Added FnSelect function to JSONable.rb

Added a small function to allow the use of the Cloudformation intrinsic function Fn::Select.

Example usage:

Parameter("ElbSubnets") {
    Type  "CommaDelimitedList"
    Default "subnet-12345, subnet-54321"
  }

Resource ("ElasticLoadBalancer") {
   Type "AWS::ElasticLoadBalancing::LoadBalancer"
   Property("Subnets", [ FnSelect("0", Ref("ElbSubnets")), FnSelect("1", Ref("ElbSubnets")) ] )
...
...
}