Integrate Coveralls.io for coverage tracking

[singingwolfboy]

You'll need to sign in to coveralls.io and enable it for this repo.

[bgreen-litl]

Thanks! I went ahead and enabled the repo in coveralls.

One thing: the [Coverage Status] bit needs to go in the Makefile under the docs target because of the way we generate the README from the python docstring. This may not be conventional but it allows us to keep the module docs and the github docs in sync by running 'make docs'.

[singingwolfboy]

@bgreen-litl Fixed.

[coveralls]

Changes Unknown when pulling a4b8530 on singingwolfboy:coveralls into * on litl:master*.

[bgreen-litl]

Thank you.