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