Adds configuration options to use Google Drive content manager

Adds the key contentmanager_js_source to webapp_settings that allows for specifying the content manager JavaScript source file.  Also adds a NotebookManager subclass, ClientSideNotebookManager, which does minimal logic.  This class is used when the JavaScript content manager doesn't use the Python notebook manager, but rather implements that logic client side, as is the case for the Google Drive based content manager.

A sample command line that uses the Google Drive content manager, and the ClientSideNotebookManager, is

ipython notebook --NotebookApp.webapp_settings="{'contentmanager_js_source': 'base/js/drive_contentmanager'}" --NotebookApp.notebook_manager_class="IPython.html.services.notebooks.clientsidenbmanager.ClientSideNotebookManager"