#73: sideboard now just runs out of one big virtualenv

This ended up being pretty straightforward, even including documentation updates.

Everything basically works exactly the same.  There are some moderate performance benefits at startup, mostly when running ``py.test``.

The MAGFest code is working fine with the new system with no changes needed, which is a good sign.
This is the main thing that I added.  So instead of going through and re-initializing all of the venvs we basically just go through and run ``python setup.py develop`` in all of the plugin directories.

The main reason to do it this way instead of using ``pip`` is that this is more reliable on Python 3, where making a virtualenv doesn't give you ``pip`` automatically and you need to install ``setuptools`` manually, at which point this will work but ``pip`` would not.

Come to think of it, I should probably update the docs with instructions on how to do this Python 3 stuff.  Maybe after I move to Python 3.4.