Reddit provider

Reddit takes its sweet time, on the order of tens of seconds, to respond. I'm not sure of the practical use of this but here it is.

Reddit asks in the rules to provide a descriptive user-agent. Generic user-agents like "python" (requests uses a generic user-agent) are severely rate limited. I added a config option for the end-user to provide an application/version specific user-agent. It defaults to "velruse" currently but it would be best if the Velruse version number was included. I didn't want to munge around with setup.py to be able to provide that information so I left it as "velruse".

I tested on 2.7 and 3.3.

Let me know if something is amiss.