Proof of concept coverage implementation
Merge remote-tracking branch 'origin/pr/77'

Conflicts:
	Gemfile
Use Istanbul instead of JSCoverage (WIP).
Just use assets from the projects `app/assets` directory.
Update readme to use the Sprockets rake task to clean the assets.
Add coverage options to the cli and the plugin.
Update travis options.
Add coverage cli options to the README.
Add coverage report generation.
Make all coverage functions work.
Change coverage merge and text reporter.
Consistent option names in the specs. (Closes #101)
Fix spelling error.
Update CHANGELOG.
Add server timeout hint for Travis CI.
Avoid partial path match.
Add istanbul to the highlight section.
Fix coverage specs.
Merge branch 'coverage'