Add unit tests

[jonblack]

There are no tests at the moment which makes refactoring difficult. The tests would ideally run on the host rather than on an Arduino to make continuous integration easier.

[s-celles]

Ensure that code can be compiled successfully (which is currently the case) could be a first step.
So when a PR is send it will ensure it compiles but you are right unit tests are a good practice.

[s-celles]

Here is a first example of a .travis.yml file

language: python
python:
    - "2.7"

# Cache PlatformIO packages using Travis CI container-based infrastructure
sudo: false
cache:
    directories:
        - "~/.platformio"

env:
    - PLATFORMIO_CI_SRC=examples/lcd_nav
    - PLATFORMIO_CI_SRC=examples/current_item
    - PLATFORMIO_CI_SRC=examples/current_menu
    - PLATFORMIO_CI_SRC=examples/lcd_nav
    - PLATFORMIO_CI_SRC=examples/led_matrix
    - PLATFORMIO_CI_SRC=examples/led_matrix_animated
    - PLATFORMIO_CI_SRC=examples/pcd8544_nav
    - PLATFORMIO_CI_SRC=examples/serial_nav

install:
    - pip install -U platformio

    #
    # Libraries from PlatformIO Library Registry:
    # missing ht1632c for led_matrix
    # missing ht1632c for led_matrix_animated
    - platformio lib -g install Adafruit_GFX  # for pcd8544_nav

script:
    #- platformio ci --board=uno --board=teensy31 --board=due --lib="."
    - platformio ci --board=uno --lib="."

I'm still facing several issues.

Despite Adafruit_GFX is installed using

$ platformio lib -g install Adafruit_GFX

(for pcd8544_nav example)

running locally this example using:

$ export PLATFORMIO_CI_SRC=examples/pcd8544_nav
$ platformio ci --board=uno --lib="."
*********************************************************************************************************************************************
Obsolete PIO Core v3.4.1 is used (previous was 3.5.0a4)
Please remove multiple PIO Cores from a system:
http://docs.platformio.org/page/faq.html#multiple-pio-cores-in-a-system
*********************************************************************************************************************************************
The next files/directories have been created in /private/var/folders/z2/1vydt2v5213482bn6yry5vn40000gn/T/tmpPprtUy
platformio.ini - Project Configuration File
src - Put your source files here
lib - Put here project specific (private) libraries

Project has been successfully initialized!
Useful commands:
`platformio run` - process/build project from the current directory
`platformio run --target upload` or `platformio run -t upload` - upload firmware to embedded board
`platformio run --target clean` - clean project (remove compiled files)
`platformio run --help` - additional information
[Thu Aug 24 16:24:25 2017] Processing uno (platform: atmelavr; board: uno; framework: arduino)
---------------------------------------------------------------------------------------------------------------------------------------------
Verbose mode can be enabled via `-v, --verbose` option
Converting pcd8544_nav.ino
Collected 27 compatible libraries
Looking for dependencies...
Library Dependency Graph
|-- <arduino-menusystem> v3.0.0
|-- <SPI> v1.0
Compiling .pioenvs/uno/src/pcd8544_nav.ino.o
Archiving .pioenvs/uno/libFrameworkArduinoVariant.a
Compiling .pioenvs/uno/FrameworkArduino/CDC.o
Indexing .pioenvs/uno/libFrameworkArduinoVariant.a
Compiling .pioenvs/uno/FrameworkArduino/HardwareSerial.o
/private/var/folders/z2/1vydt2v5213482bn6yry5vn40000gn/T/tmpPprtUy/src/pcd8544_nav.ino:13:26: fatal error: Adafruit_GFX.h: No such file or directory
#include <Adafruit_GFX.h>
^
compilation terminated.
Compiling .pioenvs/uno/FrameworkArduino/HardwareSerial0.o
*** [.pioenvs/uno/src/pcd8544_nav.ino.o] Error 1
========================================================= [ERROR] Took 1.70 seconds =========================================================

raise error (Adafruit_GFX.h is not found)

Build also fails for led_matrix and led_matrix_animated.
But that's more understandable (because ht1632c is missing) and I didn't find an easily installable version PaulStoffregen/ht1632c#1 (an Arduino or PlatformIO package).

Others examples can be compiled on Travis side without any problem.