Add support for ROS Service client

This commit adds support for generating service messages and calling ROS services. This does not implement a ROS service provider. The most of the new code written after [Services code review 1](https://github.com/acowley/roshask/pull/22) are in pkgBuilder and depFinder.
This module isn't great from a focus perspective, and this definition typified the problem. It's my fault getting it to that state, but I think now there's enough pressure that it's worth splitting things out. We should probably have a helper for messages and a helper for services, maybe they could even be in a separate module.

If it's fresh in you're head, do try to take a whack at factoring it some. Admittedly, extracting pieces will be a bit of a pain as there are several things in scope at the point where things like `genAndWriteService` are defined, but I think it's worth trying to see if things can be made more clear.