Add BucketManager class

This moves the design document manipulation functions into the
BucketManager class, which is now obtained via the new bucket_manager()
method in the Bucket class. This breaks API with older versions

Change-Id: I3947fc0fa672d1eeeaf26b3c21addb0935bb9eef
Reviewed-on: http://review.couchbase.org/41308
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>