Resource Policies

Adds support for UpdatePolicy, CreationPolicy and DeletionPolicy when composing templates.

For example:

asg := cloudformation.AWSAutoScalingAutoScalingGroup{}
asg.SetUpdatePolicy(&cloudformation.UpdatePolicy{
    AutoScalingRollingUpdate: &cloudformation.AutoScalingRollingUpdate{
        MaxBatchSize:                  2,
        MinInstancesInService:         2,
        PauseTime:                     "P5M",
        WaitOnResourceSignals:         true,
    },
})

template := &cloudformation.Template{
    Resources: map[string]interface{}{
        "AutoScalingGroup": asg
    },
}