[Proposal] Coveralls Integration with Travis CI

[Coveralls.io](https://coveralls.io) can be used to expose test coverage info on a webpage and as a badge.

An example of the webpage (using my fork):

> https://coveralls.io/r/ebollens/laravel-framework?branch=master

An example of the badge (also using my fork):

> [![Coverage Status](https://coveralls.io/repos/ebollens/laravel-framework/badge.png?branch=master)](https://coveralls.io/r/ebollens/laravel-framework?branch=master)

This pull request will do the following:

* Adds `php-coveralls` as a dev dependency in `composer.json`
* Adds `--coverage-clover build/logs/clover.xml` to the PHPUnit invocation in `.travis.yml`
* Adds `php vendor/bin/coveralls` invocation after script in `.travis.yml`
* Adds badge referencing `master` branch in `readme.md`