New gateway info and multimaster test framework

Gateway info has been moved to a latched publisher (easier) and a utility called SubscriberProxy to make it easy to use subscribers in the same way as a service client.

Also we now have a decent test framework for multimaster in rocon_test. It follows the same workflow as rocon_test, but uses rocon launchers instead of ros launchers. It also has a cmake module so you can add_rocon_tests inside the cmake. This is to solve the issue #51.