Add DoorkeeperObjectRef, DoorkeeperBridge, DoorkeeperBridgeAsana

Summary:

• DoorkeeperObjectRef is a convenience object to keep track of <applicationType, applicationDomain, objectType, objectID> tuples.
• DoorkeeperBridge provides pull/push between Phabricator and external systems.
• DoorkeeperBridgeAsana is a bridge to Asana.

Test Plan:
Ran this snippet and got a task from Asana:

{P871}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6273