Command line entry point to list running notebook servers
marked now adds ids to header tags
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
initial test for the embedding
minor js test fix
Merge pull request #4799 from ivanov/minor-jstest

minor js test fix
remove six usage (unnecessary starting python 2.6)
download should work also for python3
Qt frontend: Handle 'aborted' prompt replies.

On some rare occasions, the execution request for prompt
is aborted by the kernel (due to an error in another execution
request), which is not handled gracefully by the ipython frontend
raising `KeyError: 'execution_count'`
This commit simply handles 'aborted' prompt requests by retrying it.
Centering equation in markdown and code cells.
Added new structure (input prompt and inner cell) to markdown cells.
Restructuration of null.tpl and addition of inner_cell classes.
Rename of input groups and use of self.block for e_in_prompt.
Deleting html clases for raw and unknown type cells.
Simplification of the logic.
Coming back raw cell to null template.