Use find -print0 | xargs -0 for cleaning up generated files.
Cleaned up the Makefile.

- Updated coverage-annotate target to generate HTML output instead.
- Updated list of PHONY targets.
coverage Makefile target chains return value of run_tests.py

This way `make coverage` can be used as the testing command for CI.
Ignore coverage-html directory.
Travis is now going to show coverage results as well.
Show error and failure traces in show_results()
Merge branch 'customize-test-results' into dev