Make it a little easier to use Magithub from other packages

[vermiculus]

magithub-repo can now take a repository name instead of a full/sparse repository object. This means I can do fancy things like this:

(magithub-repo "vermiculus/magithub")

and magithub-repo will recognize this as a valid repository identifier and piece it out into a more traditional magithub-repo call.

This comes in handy when using functions that require a repository context. Something like this...

(magithub-issue-new (magithub-repo "your/project"))

...can easily be a good way to gather bug reports inside emacs.