Add a way to pre-fetch notebooks onto an image at launch.

User may configure a git repository URL which will be
cloned by the instance at /home/ubuntu/notebooks (accompanying
changes in notebookcloud_ami/nb_url branch implement the
instance side of this bringup)

added nb_url attribute to account object

append nb_url to user_data for transmission to instance

during instance bringup monitoring in mainscreen, check
the urlfetch result for 500, which signals a problem cloning
the notebook repository.

fixed bug in main.py that caused a new account to be created
when an existing was being updated (self.user)

changed mainform to prepopulate form values from account object
so user knows what is configured.

added optional notebook url field to mainform

on mainscreen, report current configured notebook and ssh key
settings in the launch info text area, so user knows what will
be launched by the button.
Merged with welch/nb_url.