Add Conduit method for querying user status

Summary:
I want to use this to warn user if he specifies reviewers that are away.

We can also implement a general query method but I think that this usage is the
most useful not only for me but also in general case.

Test Plan:
Call the method for user which is away and which is not away.
Add user status through Conduit.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2492