Add support for GitHub pagination and ratelimiting

[gene1wood]

Create a new GitHubClient class which extends the Client class by modifying
the request method and creating four new methods

• Establish a new paginate parameter to the constructor
• Modify the request method to
  • check if there aren't any allowed requests remaining due to ratelimiting
    and if so wait the designated amount of time before making the request.
  • if the paginate parameter is true, look in the payload of the response
    page for a link header and if found
    extend the fetched data by calling the get_additional_pages method
• Create the no_ratelimit_remaining method to check if there are no
  remaining requests allowed
• Create the get_additional_pages method which calls the
  get_next_link_url method to determine if there are additional pages. If
  so, fetch the next page and recursively continue fetching pages until the
  last page is fetched
• Create the get_next_link_url method which parses the link header and
  either returns the next page if there is one or an empty string if not
• Create the ratelimit_seconds_remaining method which returns the number
  of seconds remaining until the ratelimit is cleared

This new functionality can be used like this

>>> from agithub.GitHub import GitHub
>>> g = GitHub(token='token', paginate=True)
>>> status, data = g.issues.get(filter='subscribed', foobar='llama')

Based on conversations with jpaugh and nnja indicating we're going to redo the unit tests from scratch, I removed my in progress unit tests from this branch and figured I'd submit it now to make it available to folks despite the lack of unit tests.

This fixes #10