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