Run code coverage analysis with coveralls

This PR updates the Travis-CI config to integrate with [coveralls service[(https://coveralls.io/) for code coverage analysis.  Also adds coveralls requirement for local development.

To see what results look like, see [my first run of this on rasterio](https://coveralls.io/builds/1293566).

You will need to setup a primary account in coveralls service linked to github and enable this repo (interface is similar to Travis-CI).  Consider adding a badge to ```readme.rst```

We may want to tweak the files to exclude; right now everything in ```rio``` is being analyzed even though there are no tests there.

While there are other code coverage tools out there, this provides a nice web interface that may help motivate contributors to write more tests...