Implement HoundCI for linting PRs

[sindrehan]

HoundCI is a continious integration service that will lint source code and put comments directly in Pull Requests.

Looks like a good service and seems easier to deal with than running roslint as a part of the test rig in Travis. The primary benefit with a service like this is that when review a PR we can focus on the things that actually matter, and let a bot handle the bikeshedding.

However, it does not support C++ out of the box, but this blog post walks through how one adds support. Seems fairly straight forward.

Here's an example on how the bot comments a PR (from Home-assistant):