Merge pull request #4690 from takluyver/tests-subproc-streams-option

Option to spew subprocess streams during tests
Add HTTP handlers for nbconvert
Add tests for nbconvert HTTP service
Add menu entries for getting converted views of a notebook
Add MIME types to nbconvert exporters
Add option to download as reST
Condense raw_mimetype and mime_type traitlets into output_mimetype
Capture server logs when running notebook tests
Make output_mimetype accessible from the class.

Traitlets are only accessible by instantiating the class. There's no
clear reason that this needs to be a traitlet, anyway.
Add GET /nbconvert to list available formats
Describe nbconvert in notebook UI for whatsnew
Separate listing nbconvert exporters to /api/nbconvert
Download as always starts downloads in new window/tab

This is a slightly worse user experience if it succeeds, because the new
tab flashes up before closing again, but it will let us display an
informative error page if it fails, without navigating the user away
from the interactive notebook view.
Fix notebook tests w/ new stream capturing API.