RDS additions to support hibernating a db instance

Hibernate in this case means to stop an RDS instance and then restart it
later and have the new instance be identical to the original. Can mean
big savings in aws charges. e.g. stop a db at 7pm in the evening and
then restart it at 7am in the morning - halves your costs.

See Wiki for example usage - RDS Example - stop-db.pl and start-db.pl.

Following actions have been added. Most other RDS actions have been
modified.
modify-db-instance
describe-db-instances
describe-db-snapshots
copy-db-snapshot
delete-db-snapshot