aws_db_parameter_group (or AWS?) does not support removing previously
defined parameters.

I thought I'd give "taint" a spin and force the parameter group to be
recreated.  terraform tried to delete the parameter group but failed as
it's still in use.

This change allows terraform to migrate the RDS instance to the new
parameter group before destroying the old.

Accessing aws_db_subnet_group.this when its count = 0 is an error, so we
need to instead access it via splat syntax and then apply a default value
for when the resulting list is empty.

Added iam_database_authentication_enabled and license_model model

Added complete example for Postgres RDS

support final_snapshot_identifier

Update README

Add enhanced monitoring

Using `storage_encrypted` to set the db_instance.storage_encrypted

db parameters can now be passed to db_parameter_group module