Add coverage support to travis via coveralls.io
It might be nice to add support for coverage info via [coveralls](http://coveralls.io). [Here](https://coveralls.io/info/features) are some of the features. Looks like [this Python interface](https://github.com/coagulant/coveralls-python) is slightly more actively developed and would be the way to go. Not sure if this is worth the pain of setup, but I always find coverage info helpful after running the full test suite and it can be useful for prioritizing areas of the code base. I know pandas's travis config doesn't run the whole ./test.sh but I believe coverage.py can be told what dirs to cover. Down/(up?) side: this will add to travis's running time, but only if tests are successful.
