Add a pbr compatible setup

Add a setup.cfg and setup.py and fill in those files
so that gertty can be installed easily.

Adjust gertty.py to have a main function that
can be referenced as a console entrypoint during
installation.

Also adjusted a bunch of imports that were failing after
installation occurred (but now they should not fail since they
correctly reference the gertty module namespace).

Change-Id: I532ced51583b26300ba9a2efb97f8e41bc69ec8d