Add coveralls.io support

Uses [grunt-karma-coveralls](https://github.com/mattjmorrison/grunt-karma-coveralls) to generate coverage statistics and pipe them to [coveralls.io/r/tagged/infinite-scroll](https://coveralls.io/r/tagged/infinite-scroll).

New Badge
[![Coverage Status](https://coveralls.io/repos/tagged/infinite-scroll/badge.png?branch=coveralls)](https://coveralls.io/r/tagged/infinite-scroll?branch=coveralls)