Support for List and Map

I'm  purposefully returning mutable objects here to keep the behaviour as similar to Scrooge as possible.

It's not my preference to have mutable data structures. However, that is how Scrooge does it and I wanted their behaviour to be as similar as possible and not stray to far from the beaten path.

Humbug is there as a work around to the issue of having to support large thrift structs. At this stage it is not a better implementation of a Thrift to scala code generator.