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 busy/idle around every message

not just shell
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