Write notebook server info file in security directory
Test for writing and removing server info files
Fix writing server info files on Python 2
warn when notebook is started in pylab mode

because it is always a bad idea
mention that `ipython notebook --pylab` will be disabled
Command line entry point to list running notebook servers
Make names in JSON more consistent as per @ellisonbg's suggestion.
be conservative about kernel_info implementation

in case kernels don't comply with the message spec by leaving out the language key.
Merge pull request #4797 from minrk/safer-guiref

be conservative about kernel_info implementation
Merge pull request #4772 from takluyver/nbserver-files

Notebook server info files
Merge pull request #4788 from minrk/warn-pylab-notebook

warn when notebook is started in pylab mode
Merge pull request #4799 from ivanov/minor-jstest

minor js test fix
adding visual play button
adding visual interrupt button
add toolbar highlighting on hover
try to have buttons render properly on nbviewer