Set axis background to transparent in inlinebackend

Fixes #5727, related to #4608
Updated license to include CC BY 4.0 for documentation, examples, and other materials
Added examples to BSD, and added links to online versions of CC-BY
Moved the list of links of the CC-BY licenses below (I could not get RST to render correctly)
Check for pids when listing nbserver processes
Use check_pid from utils in IPython.parallel
pin tornado to < 4 on travis js tests

Current phantom uses an old webkit
that implemented draft76 of web sockets.

Tornado 4 drops support for draft76.

This doesn’t affect any real browsers,
as far as I can tell.
Merge pull request #6143 from minrk/tornado-vs-casper-cagematch

pin tornado to < 4 on travis js tests
Add 'mistune' markdown parser as an option for markdown2html
Add highlighting of GFM code blocks with mistune
Make mistune a requirement for nbconvert
Add deps for nbconvert to HTML for 'notebook' target

Also, nbconvert no longer requires Sphinx.
Integrate pass-through of mathematical expressions with markdown rendering
Version dependency on mistune
No need to install nodejs on Travis
Add support for \begin foo \end math blocks
markdown2html test doesn't rely on pandoc
Rename mistune renderer class
mistune rendering is the default
Fix code block rendering when lexer not known