add coverage testing

[belevtsoff]

[cpcloud]

What about using coverage.py? This creates another dependency :(. Nose has a --with-coverage option that uses this module.

[cpcloud]

I tried this and it performs coverage on all of the included libraries as well, which makes for some annoyingly long output.

[btel]

coverage.py is definitely the tool to use. There should be a way to exclude external dependencies. You might take a look at: http://nedbatchelder.com/code/coverage/source.html