Preliminary Conduit info method for Maniphest tasks.

Summary:
Provide a simple maniphest.info method for fetching info about a Maniphest task, given its ID.

Test Plan:
Tested via the web console, and it appeared to work fine. This is preliminary though, but I plan to use it for adding 'Txxx' to phabot.

Reviewers:
epriestley

CC:

Differential Revision: 540