Add support for "HELLO"

Change-Id: I2f209ad2ec313c68e2d90aac8a5371a80d9a30c6
Reviewed-on: http://review.couchbase.org/31654
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>