Multi-dir webservice design

## Multi-directory support

This PR is a web-service redesign for the notebook that allows multi-directory navigation. This was proposed in [IPEP 16](https://github.com/ipython/ipython/wiki/IPEP-16%3A-Notebook-multi-directory-dashboard-and-URL-mapping) and will not be released in 1.0. 

#### Things that have changed:

* The notebooks are not longer identified by a UUID. 
* The URL includes the path to the notebook and the names of opened notebooks
* New webservice APIs are introduced... 'sessions', 'notebooks', 'kernels', and 'contents' (not quite finished).
* A new mapping manager, called a ```session```, is used to link notebooks and kernels... kernels are not longer entangled with the notebook_id
* User can navigate through sub-directories by adding the files in the URL 'path'.

See the web-service documentation in the GIST [here.](https://gist.github.com/Zsailer/5767194)