A more comprehensive implementation of rds vpc security groups

@toastdriven and @danielgtaylor, please take a look at this implementation based on @dialt0ne's original pull request.

One feature lacking from the current develop branch on boto is the ability to read the current security groups
of an RDS.  dialt0ne's implementation allowed this, as well as the vpc_security_group parameter in modify_ and
create_dbinstance.  however, it was lacking unit tests.

I've combined danielgtaylor's unit tests and amended them to test the reading of vpc security groups from
an existing DB instance.

consider replacing the current version with this very similar, but more comprehensive, one.