support DBParameterGroup resource

Adds support for the RDS DBParameterGroup resource. This resource is
atypical in a number of ways:

1) You cannot set parameters on the parameter group on creation.
   Instead, you need to create the parameter group and then call
   ModifyDBParameterGroup and pass in the parameters that you want to
   include in the parameter group.
2) When modifying parameters in a parameter group that is associated
   with a DBInstance, only some of the parameters are changed
   immediately. Other parameters require a reboot of the EC2 instance the
   DB is running on in order to actuate the change.
3) When creating a parameter group, you need to wait around 5 minutes
   before specifying it when creating a DB instance.

Author: jaypipes