Add support for Python dependencies defined in a requirements.txt file

This pull request adds support for Python dependencies.

There are also some basic terminology changes to be more generic when dealing with gem vs. distribution. We stopped short of cleaning up all the areas that mention gems or bundler specifically.

Python libraries are likely to store their licenses remotely on PyPI (https://pypi.python.org) and as such this PR also adds support for pulling down those remote licenses. This should be effectively compartmentalized so that no Ruby project will ever have to see it.