Allow Ref types for a Statement's Resource attribute

Hi,

Currently it seems like `awacs.aws.Statement`'s Resource attr can only be a `list` (https://github.com/cloudtools/awacs/blob/master/awacs/aws.py#L143)

It would be useful to allow passing in a `troposphere.Ref` object because then you can have a CommaDelimitedList parameter which contains the resources you want to create the policy for.

I guess this could be done with a callable, like the `Effect` type check in `Statement`, although it feels like there could be a more general solution that could be useful for all types, not just Statement.

Thanks

Start in awacs/aws.py at the Statement Resource attribute around line 143, then compare its type checking with the callable-based Effect check. Determine how a troposphere.Ref containing a CommaDelimitedList should be accepted while preserving existing list handling. Done means Statement can construct a policy with a Ref-based Resource without rejecting valid existing inputs.

Written by the indexing model from the issue text.