build: add make targets for reporting coverage

To avoid the need to create a pull request to get Coverage report.

Usage:
  $ make report-coverage-html

Why `coverage3`? Because my Fedora box runs coverage with python2.7.

Taken over from:
  https://github.com/jfilak/autorizator/blob/master/Makefile

Author: filak-sap

Makefile

@@ -1,4 +1,10 @@
 PYTHON_MODULE=pyodata
+PYTHON_BINARIES=
+PYTHON_MODULE_FILES=$(shell find $(PYTHON_MODULE) -type f -name '*.py')
+
+TESTS_DIR=tests
+TESTS_UNIT_DIR=$(TESTS_DIR)
+TESTS_UNIT_FILES=$(shell find $(TESTS_UNIT_DIR) -type f -name '*.py')
 
 PYTHON_BIN=python3
 
@@ -13,6 +19,13 @@ FLAKE8_BIN=flake8
 FLAKE8_CONFIG_FILE=.flake8
 FLAKE8_PARAMS=
 
+COVERAGE_BIN=coverage3
+COVERAGE_REPORT_ARGS=--skip-covered
+COVERAGE_CMD_HTML=$(COVERAGE_BIN) html
+COVERAGE_HTML_DIR=.htmlcov
+COVERAGE_HTML_ARGS=$(COVERAGE_REPORT_ARGS) -d $(COVERAGE_HTML_DIR)
+COVERAGE_REPORT_FILES=$(PYTHON_BINARIES) $(PYTHON_MODULE_FILES)
+
 all: check
 
 .PHONY=check
@@ -24,10 +37,23 @@ lint: doc
 test:
 	$(PYTHON_BIN) -m $(PYTEST_MODULE) $(PYTEST_PARAMS)
 
+.coverage: $(COVERAGE_REPORT_FILES) $(TESTS_UNIT_FILES)
+	$(MAKE) test PYTEST_PARAMS="--cov-report= --cov=$(PYTHON_MODULE)"
+
+.PHONY: report-coverage-html
+report-coverage-html: .coverage
+	@ echo "Generating HTML code coverage report ..."
+	@ $(COVERAGE_CMD_HTML) $(COVERAGE_HTML_ARGS) $(COVERAGE_REPORT_FILES)
+	@ echo "Report: file://$$(pwd)/$(COVERAGE_HTML_DIR)/index.html"
+
 .PHONY=check
 check: lint test
 
 .PHONY=doc
 doc:
 	$(MAKE) -C docs html
 	@ echo -e "\nOpen: file://$$(pwd)/docs/_build/html/index.html"
+
+.PHONY=clean
+clean:
+	rm --preserve-root -rf $(COVERAGE_HTML_DIR) .coverage