pull request for issue #8, Add a way to pre-fetch notebooks onto an image at launch

summary:
    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)
    
commit notes:
    these build on the ec2_key branch changes.

    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.