Fixes #1 - add --rate-limit command

[GregSutcliffe]

This is my first pass at any real Python, so go easy ;)

Simple switch to show the current status of the GitHub API. Example output:

greg@bicho$ pullpo -u root -p 12345678 -d pullpo --gh-token 1234 owner repo --rate-limit
Remaining rate limit requests: 5000
Rate limit requests reset at:  2016-05-26 14:07:04

Sadly, although this does not need to query the db or a repo, I'm struggling to make the other arguments optional. This is not a major issue, since you're usually querying the API as part of a wider Pullpo update, so simply adding --rate-limit to a command from your history is enough.