GoogleOAuth2 support (login with --auth option)

This is a not yet completed fix for OAuth2 (https://github.com/mher/flower/issues/229):

 - some hardcoded values should be put in settings.
 - login always redirect to home page
Display error message for failed tasks in result

It's nice to be able to look at why tasks are failing at a glance
without having to drill down into each individual one
Read OAUTH2 credentials from environment variables
Rename Workers tab to Dashboard
Get dashboard info from events
Rename dashboard columns
Add worker average load column to dashboard
Add worker label to worker name
Flower requires Celery 3.1.12
Add test requirements
Flower requires Celery 3.1.7
Show worker's system info
Show prefetch count
Merge branch 'master' of github.com:mher/flower into dashboard
Fix api doc markup
Don't print debugging states
Update worker queues
Fix merge conflict
Fix incomplete merge conflict
Refactor dashboard