Drive

Starting support for Google Drive.
The Google Drive interface combines saving a revision and marking it as a checkpoint.  Since we are using Drive's revisioning to do checkpointing for notebook, it is better to have one call do both saving and checkpointing.  If two separate calls were made, one to save and one to checkpoint you would end up with two revisions with the only difference being that one would have the retain flag set.  This matches the semantics of the user interface of notebook so I added the flag to the model on saving so that the checkpoint could be created in the same operation.