Add update_settings and channel_changed functions - #20

This resolves #10 by allowing Python modules to react when the number of channels, sample rate, and/or set of selected channels changes. The new update_settings function receives the number of channels and sample rate whenever updateSettings is called in the C++ plugin, and channel_changed is called whenever the editor's channelChanged function gets called.

Other things:

• The Cython compiler was interpreting plugins as Python 2 rather than 3 (I believe due to one of my earlier PRs - sorry), so I changed all the setup.py files to fix that.
• Added the resetPlugin function to make it so that the function pointers are always null unless there is a plugin loaded. I'm thinking this could be useful in a future PR to allow unloading plugins.