Document how to run tests

[JamesGuthrie]

There are a lot of tests for pyretic but it's unclear how they should be run. Using a test runner like nosetests fails and there is no clear tests.py file which can be run (and executes tests). Could you please document how the tests are to be run? Additionally, it might be cool to have a travis automatic build which can be used to evaluate pull requests.

[joshreich]

The testing code has evolved quite a bit over time. I'm honestly not entirely sure what most of it is for anymore (though my suspicion is that it's mostly out of date). When I last developed, the test I was using was $ py.test pyretic/tests/tests.py

Srinivas has been the most active developer lately. Srinivas, can you provide James w/ brief documentation on the current set of test you are running?

[JamesGuthrie]

Thanks for the update, unfortunately py.test pyretic/tests/tests.py does not successfully run the tests but it put me on the right path the get the tests working again. I have opened a PR with the changes.