Adds a Google Drive base Content Manager, and moves "load_notebook" to content manager

Adds a Google Drive based content manager, and a system to specify this content manager through IPython's configuration system.  Also moves load_notebook function to ContentManager.

Sample command line invocation of IPython that uses the configuration for using this content manager:

    ipython notebook --NotebookApp.webapp_settings="{'contentmanager_js_source': 'base/js/drive_contentmanager'}" \
        --NotebookApp.notebook_manager_class="IPython.html.services.notebooks.clientsidenbmanager.ClientSideNotebookManager"