DUI: tooltip for addons. Part 2

This PR should be merged after [that one](https://github.com/Benglin/Dynamo/pull/183).
This PR makes possible to generate input and output parameters for custom node from XML file.

Reviewers
@Benglin ,@vmoyseenko, please take a look.
I would have hoped this condition gets moved into `TryLoadDocumentation`, but that's a small thing.