Implement local nbdiff server

This commit allows you to use Nbdiff as a standalone application, instead of an extension of the IPython Notebook. Nbdiff is now run on its own local server.  

To run: 
-You must first be on the command line in a git repository with a notebook that has a merge conflict.
-Type nbmerge in the command line, this starts a flask server. 
-Go to 127.0.0.1:5000 in your browser to see the Nbmerge UI. 

To do:
-Launch the browser automatically from the nbmerge command
-Currently not able to save merged notebook from the UI
-Not currently implemented for diffs
-No tests yet