Created TransformerHelper

It stands between Transformer and TransformListener. It decouples the
2 functionalities of the TransformListener class: listen to transforms
and provide helper functions to transform data.

This seems trivial, but in my case it's very important to decouple the 2. My code is designed this way: libraries that can be used offline, and online node that wrap those libraries in a communicating ROS framework. In the offline case I really want to use the helper functions from TransformListener to transform my data, but I don't want to be listening to incoming transforms (transforms are pushed manually inside the transformer).

I tested it with my large code base. It can be used as is. And I tested the decoupling, it works. So it seems to be pretty safe modification to me.