adding google drive rewrite provider

[bollwyvl]

A dropbox-style provider based on #427.

Can take the two kinds of links i found in drive (the "share" UI and the "get link" UI).

Tests passing currently blocked by #481, as it uses ? params.

[bollwyvl]

Turns out there is a way to publicly host a whole directory, which enables relative links:

1. This publicly-shared folder:
   • https://drive.google.com/folderview?id=0B7d519FxJIqPfkg0Wk1XSW42cWRQMnFTdXVLVkdUNWV0c0JRX00tVGpEblF3U1FqYnJjckE&usp=sharing
2. Can be dereferenced to this link:
   • http://googledrive.com/host/0B7d519FxJIqPfkg0Wk1XSW42cWRQMnFTdXVLVkdUNWV0c0JRX00tVGpEblF3U1FqYnJjckE/
3. Which contains a link to Untitled.ipynb:
   • https://4faab7555cc0ed4cb029e9ed361eb8e38c4b85fc.googledrive.com/host/0B7d519FxJIqPfkg0Wk1XSW42cWRQMnFTdXVLVkdUNWV0c0JRX00tVGpEblF3U1FqYnJjckE/Untitled.ipynb
4. The first part of the domain can be stripped to yield this valid nbviewer link:
   • http://nbviewer.ipython.org/url/googledrive.com/host/0B7d519FxJIqPfkg0Wk1XSW42cWRQMnFTdXVLVkdUNWV0c0JRX00tVGpEblF3U1FqYnJjckE/Untitled.ipynb

On my local branch with #481 applied, this actually shows the kitty.

So while that is an awful link, it is actually pretty viable for near-term integration, though it would probably require some documentation/screecasting/bookmarkleting, as it requires URL hacking... or index parsing, which I don't think is worth it. However, given that http://github.com/jupyter/jupyter-drive is a thing, this could be a very powerful way indeed to work on notebooks for publication with a team.

[bollwyvl]

Let's not do this!

http://googleappsdeveloper.blogspot.com/2015/08/deprecating-web-hosting-support-in.html

[rgbkrk]

Of course...