Add coverage report :lipstick:

Adds support for code coverage reporting via Istanbul and Coveralls.

 - Add [istanbul] deps
 - Use test helper for setting NODE_ENV=test
 - Split test suite into acceptance and unit tests
 - Cleanup and update Makefile targets accordingly
 - Update npm scripts, add `debug` target
 - Update travis-ci targets
 - Add [coveralls] integration

[istanbul]: http://gotwarlost.github.io/istanbul/
[coveralls]: http://coveralls.io