R1:4579f23f63a8

Add a "maniphest.update" Conduit method

Summary:
- Add maniphest.update
- Add support for auxiliary fields to maniphest.createtask

Test Plan:
- Created tasks with maniphest.createtask
- Updated tasks with maniphest.update

Reviewers: btrahan, jungejason, zeeg

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1330