Update __init__.py

Support for VPC Security Groups in RDS deployments
Rename vpc_security_groups_ids -> vpc_security_groups for consistency with security_groups option; Modify behavior to allow passing in SecurityGroup instances from EC2.
Documentation fixes
Add unit tests for vpc_security_groups when creating RDS database instances.
Merge pull request #1683 from danielgtaylor/rds-vpc-sg

[RDS] VPC Security Group Support. Fixes #1683, #1602.