Provide a basic maniphest.createtask method via Conduit

Summary: This could be a lot fancier but let's see what else we need. Also fixed
some bugs with maniphest.info.

Test Plan: Used the Conduit web console to create some tasks with different
values.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 824