Add support for ROS Service client

[rgleichman]

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 are in pkgBuilder and depFinder.

[acowley]

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.

[acowley]

Any thoughts on dealing with the Travis build failures? Ideally, Travis could still run the tests, but I'd be fine with only a subset of tests running on Travis while the rest require a bit more setup.

[rgleichman]

Yes, I think Travis should only run a subset of the tests. This would mean it would run cabal test testexe instead of cabal test. Ideally, all of the tests (including service and topic tests) would be automated too.

[acowley]

Merged!