Make it possible to put the site in maintenance mode
By specifying the UTC date from which the maintenance shoulds start,
developers can put the site in 'maintenance' mode.
Also redirect to /maintenance if we get a CapabilityDisabledError.
Patch by: Sverre Rabbelier
A list of bugs is available in the Google App Engine SDK project on Google Code.
The issue tracker is at http://code.google.com/p/googleappengine/issues/.