Adding docstring to NotebookHandler.get.
Handle notebook downloads through the /files URL.
Fully removing .py file upload.

In the future all uploads should just use the contents web service
and we should support any type of file.
Improving tests and setting of Location header.
Merge pull request #4373 from ellisonbg/multidir-review1

Multidir review1

A random set of review related changes for the multidir branch.