Route53 Zone.update_record() fix [spulec/moto#83]

 * Zone.update_record() issues what is essentially a batched update to
   the AWS Route53 API: It's a DELETE followed by a CREATE.  This fix
   allow moto to handle that batched update.
Updated test_route53.test_rrset() for batch update

 * This should provide a test for [spulec/moto#84].
 * Prior to deleting the existing A record, it sends a batch update that
   includes DELETE and a CREATE, which is what boto performs when
   updating a ResourceRecord.
Merge pull request #84 from vrivellino/master

moto.route53.responses.rrset_response() fix for spulec/moto#83