break apart test dependencies and runtime dependencies

[aoberoi]

things to consider:

Travis runs pip install -r requirements.txt. this means we may need to add a test_requirements.txt file and make it also install that

the requirements.txt right now doesn't do anything in particular, just lets setup.py pick up dependencies. i was under the impression that this is better practice for libraries unless specific versions are required.

Tox is run to do testing locally, and tox.ini defines dependencies too. Right now it only declares nose as a dependency because it needs to run the nosetests binary. It works on the assumption that the sbin build that is already pip installed into the individual virtualenv has all the other dependencies that this library needs.