Recognize the HELLO command code

This will still return not supported, but it's better than crashing

Change-Id: I26a786f95c41369a560049a56d424935ac880c95
Reviewed-on: http://review.couchbase.org/37614
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>