Add HELLO support

This only adds the barebones 'HELLO' support. It does not yet support
datatype handling.

Change-Id: Iefa263656f8d2299ce28e7af4d60742807036292
Reviewed-on: http://review.couchbase.org/37600
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>