rosrt: added Xenomai support (#6)

Xenomai support added using the native Xenomai API. Added three wrapper
classes: rosrt::mutex, rosrt::condition_variable, and rosrt::thread,
which wrap xenomai functions or boost classes depending on the platform.
Tests have also been updated to work on Xenomai, because a loop without
sleeps in a Xenomai real-time thread can starve the system.