Add packaging goo (move main()); make default config path more flexible.

Now you can run $PREFIX/bin/rfmonitor (which setup.py creates for you), or pretty much equivalently, python -m monitor.app .

The configdir substring has changed to etc/rfmonitor; monitor alone is too generic. We now prioritize config file locations, and try to figure out if /usr/local/etc or /etc should be the system prefix. We prioritize ~/.local/etc if it exists.