Allowing setting user status

Summary:
I will use it for highlighting users which are not currently available.

Maybe I will also use it in the nagging tool.

I don't plan creating a UI for it as API is currently enough for us.
Maybe I will visualize it at /calendar/ later.

I plan creating user.deletestatus method when this one will be done.

Test Plan:
storage upgrade
Call Conduit user.addstatus.
Verify DB.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2382