JCBC-524: Adding support for custom transcoders.

Change-Id: I631194c5b1bd58894fe24dd34001e1e984af83fd
Reviewed-on: http://review.couchbase.org/40829
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>