Convert to context managers

This pull request turns both the cursor and connection objects into context managers, so they can be used as described in https://github.com/BradRuderman/pyhs2/issues/1

As something of a side effect, it also switches the setup.py to using setuptools (vs distutils), which makes install_requires work. This is not an essential part of the pull request, but it made my development much easier, and I would suggest it going forward. That said, if that doesn't work for you for whatever reason, I'm more than happy to remove that change.

Thanks for your time!