Implement a GitHub importer

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

```python
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.