rds: add describe snapshots
rds: add DB restore from snapshot
Merge branch 'feature/rds-snapshot-restore-method' of github.com:yudoufu/goamz into yudoufu-feature/rds-snapshot-restore-method

Conflicts:
	rds/rds.go
	rds/responses_test.go
Merge branch 'yudoufu-feature/rds-snapshot-restore-method'