Multiple Checkpoint Support and Beginnings of Google Drive Support

These changes implement multiple checkpoint support for notebooks.  There is a new menu item "Delete a Checkpoint" which allows for checkpoints to be deleted.  Checkpoints are now serialized with a simple number.  New checkpoints get a serial number one higher than the highest existing serial number.

Google Drive support can be experimented with by substituting drive_contentmanager.js for contentmanager.js.
I'm not sure we want to invent our own versioning scheme, or at least we don't want to base a Backend that deal with version directly in IPython. IIRC, we want to have the abstraction in place, and people should just be able to switch backend.