Huge refactor: Sublime Text 3 support, coverage file watching, more

* Sublime Text 3 compatibility, asynchronous whenever possible; fixes #11
* Coverage file is now watched for modifications in a separate thread, fixes #3
* More modular, defensive and testable, more logging; fixes #10 (hopefully :smile:)
* (Some very basic) automated tests, Travis CI integration (also for #10 and similar)
* File paths are always normalised now, fixes #12 (in a similar way, thanks @uehondor)