Support administrative protocol.

Add support for multiplexing the text-based admin protocol on
existing connections.

Add some basic support for some admin commands (in an extensible way).

Change-Id: Ib6c9891e22518d610b5abed75c78efcc4f23ed60
Add a method that waits for a job handle.

Job handles should be returned immediately, and are quite handy
to have in further interactions with gearman, so provide a
convenience method to wait for a job handle after submission.

Change-Id: I3b2fb3c75bf4c0618647f49386cb198e08008fab