Require param

Implementation of what was discussed in #38.

There's some other cleanup in here, but these are the key changes:

https://github.com/clearpathrobotics/rosserial/commit/a782eb684aa49df24462621d4da7e0f291892f5a#diff-085295ad0cf92aca81b1579fb9864830R306

I've also gone ahead and [documented this](http://wiki.ros.org/rosserial_server#Require_API).

Testing has been against the Grizzly MCU. Verified that when I added a topic that is not published by the MCU, the server reconfigured the client at 1Hz, waiting for that missing topic to appear.