Add support for type parameters for classes

Allow type parameters for class, ie `class Foo<Bar, Baz>`

/cc @vojtajina 
They are not types? `E` above is a type. You should be able to use:

https://github.com/google/traceur-compiler/blob/master/src/syntax/trees/ParseTree.js#L345-L357

(Which by the way needs to be updated)