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
Add support for different kernel specs to kernels REST API
Add kernel name to sessions REST API

Also, some refactoring so that the relationship between session, kernels
and notebooks is managed in the SessionManager, not in the HTTP
handlers.
Update JS for kernels and sessions APIs
Delete session rather than killing kernel in test cleanup
Fix JS syntax
Solving #5997: instances of WidgetView (that are not instances of DOMWidgetView) do not trigger widget_area.show anymore.
Remove / from route of TreeRedirectHandler.

When base_url is set, navigating to http://{url}/{base_url} returns 404
This is due to the TreeRedirectHandler only picking up
http://{url}/{base_url}/ and the trailing slash handler being set to
{base_url}/.*/.

This change will cause http://{url}/{base_url} to correctly redirect to
http://{url}/{base_url}/tree, as expected.
Add default_kernel_name class level variable
Merge pull request #6117 from thetestpeople/base_url-path-slash

Remove / from route of TreeRedirectHandler.
Merge pull request #6026 from takluyver/kernelspec-rest-launching

Kernelspecs in REST API for kernels and sessions
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.