Test greceful error-out with unsupported features.
Log message change.
Fix bug when adding multiple transmissions.

- std::vectors were being used to store raw joint data, and when new transmissions
  were added, push_back()s would (potentially) reallocate the vectors and
  invalidate already stored pointers in hardware_interfaces. We now use std::map.

- Move plugin implementations to a separate library.

- Export link libraries to the outside.

- More complete tests.
Catkin fixes.