Backups for fab deploys, tidied fab deploy, and new function for testing fab deploys locally

Created by: jogwen

Take a backup of the db and root directory at start of each fab deploy, keeping only one previous backup.

Also, further tidying of fabfile.deploy function as suggested by Hamish:

• Added check_for_local_changes() (called from overriden fablib.deploy)
• Create env.project_root using existing fablib function, not manually
• Stop calling the following from fabfile.deploy because an equivalent is called from tasklib.deploy()
  • collect_static_files
  • update_db
  • load_fixtures

Also, added a new function for testing fabric deploys on localhost via 'fab dev deploy'.