Fabric: Add stage_feature command for testing feature branches

Now it is possible to deploy a Pootle instance using a specific repo and a
specific branch, and copying a previous Pootle DB to bootstrap the deploy.

Fix bug 3004
Cleanup: use abort() from Fabric
Minor cleanups and fixes in Fabric and docs
Fabric: Add purge_stage command

Necessary for easily removing deployments performed using the stage_feature
command.