Implement a GitHub importer

[yuvipanda]

I want to be able to import things straight out of github with the following format:

from ipynb.github.someuser.somerepo.branch_or_tag_or_sha.file_path import some_function

and have that work. Relative imports should also be supported for this so you can put a package of ipynb files on github.

Also investigate how to do this without killing github's servers and have them ban us.

[yuvipanda]

Should probably not prioritize this until we get local filesystem imports solidly sorted out :D