Added support for Coveralls.io

I've set up Tweepy to publish code coverage information to [Coveralls.io](http://coveralls.io), which allows you to display a badge in the README stating your test coverage. If you'd like to use this. you'll need to create an account at coveralls.io.

See [this repository](https://github.com/z4r/python-coveralls/blob/master/README.rst) for an example of the badge.