Use pytest to run unit tests

[MichaelWedel]

This fixes #249.

The command line to run all tests manually from the top level directory is now:

$ pytest test

To get coverage information equivalent to what we have in Travis:

$ pytest --cov=lewis.core --cov=lewis.devices test

pytest warned about some deprecated methods, such as log.warn instead of log.warning and self.assertEquals that we were oddly using in some places, I've fixed those as well.

[MikeHart85]

Looks good, works as expected; merging!