Code Coverage with Coveralls

**EDIT:  I updated the history of this commit so this description no longer applies, see comments below.**

Working coverage: https://coveralls.io/jobs/970527 (With correct sources!)
Travis submits to coveralls: https://travis-ci.org/drkibitz/pixi.js/builds/16574305#L1355
Coverage badge: https://github.com/drkibitz/pixi.js/tree/dev#pixi-renderer--

## Important

*If this pull request is intended to be merged...*
 
The commit https://github.com/drkibitz/pixi.js/commit/218f831a6571db497debc023fba646dd2d2a3176 is specific to my fork, these are the things that need to be changed to be specific to pixi.js repo.

- The coverage badge is specific to my fork
- The encrypted ENV variables are specific to my fork

I can set the correct encrypted ENV variables if you want, just need 1 thing. The pixi.js repository's coveralls secret token, if feel free to send in an email *I can be trusted*. Get this token by logging to coveralls, syncing your repos, and turning on coveralls for pixi.js.