Add support for coveralls.io

Up to you, just records historical coverage data and statistics.

Examples:

https://coveralls.io/r/zomble/Slim
https://coveralls.io/builds/67915

Also the travis-ci build https://travis-ci.org/zomble/Slim/jobs/7917818 I've left the text coverage intact.
.
To activate, you'd just need to go to https://coveralls.io/ and turn it on for this repo.

Cheers.