send idle/busy on all shell messages

Instead of just those that execute code.
This means that kernel_info_request triggers busy/idle,
allowing clients to check IOPub channel status without executing code.
Set axis background to transparent in inlinebackend

Fixes #5727, related to #4608
set busy/idle around every message

not just shell
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
Solving #5997: instances of WidgetView (that are not instances of DOMWidgetView) do not trigger widget_area.show anymore.
Only allow iframe embedding on same origin.
Incompatibility notice on iframe embedding
Merge pull request #6115 from SylvainCorlay/widget-area-show

Instances of WidgetView do not trigger widget_area.show anymore.
Merge pull request #6120 from rgbkrk/no_iframe_embedding

Only allow iframe embedding on same origin by default.
remove rackcdn https workaround for mathjax cdn

cdn.mathjax.org works over https now
Merge pull request #6134 from minrk/mathjax-cdn

remove rackcdn https workaround for mathjax cdn
Merge pull request #6055 from minrk/busy-idle-always

send idle/busy on all shell messages
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