The git ref methods were mostly written to not expect the 'refs/' prefix
on ref names, even though it is included the ref.Ref value returned by
GitHub.  This resulted in ref values returned from some methods that
couldn't be passed to other methods.  In reality, ref names passed to
these methods should normally include the prefix, but this change
supports either form for backwards compatibility.

Fixes #133

This includes all of the methods mentioned in this blog post:
https://developer.github.com/changes/2014-03-18-paginating-method-changes/
as well as a handful of others I found.  Sadly, this is a breaking
change for many of these methods because it changes the method
signature, but with yesterday's API change these methods are now broken
anyway. :(

this is a breaking change, but this was only recently added so I'm not
too worried.  This also seems the best way to handle sub-namespaces on
this method.

This commit adds support for Git refs.

Listing of refs does not support sub-namespaces.