[MRG] add config validation, better input file finding

[bluegenes]

Implement some input/output checks (#110):

A. rule inputs/outputs

• categorize inputs, outputs from each rule (provide options, e.g. either raw reads or trimmed reads)
• add a "help" key to params.yml. Use this to print some helpful info about how to use the program/rule (perhaps type of input/output, or workflows that rule is currently in, etc).
• add "citation" key to params.yml. For future use printing citations of programs used in a workflow.

B. checks for input config files (+ maybe tsv files?):

• write json schema for the config.yaml that will check formatting. Catch improperly formatted yaml configs and try to be helpful. To do this:
  • script to generate yaml schema for default params for each rule file
  • aggregate these schema for included rules in order to check that input config is correct
  • use snakemake validate to check? Or otherwise validate!
  • modify "required" in schema to match current targets. To do this, generate new full schema as a process in __main__.
• for samples file (specified in config.yml), check that files can be found:
  • if absolute path, check that
  • else try from path to that yml file
  • else try from os.getcwd()
  • else provide a helpful error!
• do the above for any file specified?