Add whitelisting via procfile, use basic_consume to listen for messages.

Removes manual wait loop and uses basic_consume to register a consumer
for messages from rabbitmq. Also adds support for parsing procfiles and
using them as a whitelist for commands.

Fixes up some formatting/PEP8 issues as well, and adds defaults for
rabbitmq connection settings to make local development easier.

There are still some things missing from this; notably, the publisher can tell shove where to look for the procfile, which means that it can get shove to run commands from any procfile it has read access to. This is a problem we'll deal with in another pull request, though.