Don't automatically include api3.h in couchbase.h

Technically this is an experimental API. Some things may fail still
(such as various field name changes). We'll include it automatically
(and move the older API to a different file) once it's been marked as
stable.

Change-Id: I85d0c7090e4b4a52a01dca090c8bab8cbfeb6f5f
Reviewed-on: http://review.couchbase.org/41477
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>