Added promote command to promote read-replicas to standalone databases (http://aws.amazon.com/about-aws/whats-new/2012/10/11/amazon-rds-mysql-rr-promotion/).  Added ability for the modify command to rename a database instance via the optional new_instance_name parameter
fixed documentation & added rename example
Merge pull request #5052 from bpennypacker/promote_rename

cloud/rds module: added promote command & updated modify command to allow for renaming of database instances.