Added new virtual getName() function to constraint samplers

This is for easier debugging and plugin management. I don't know of anyone who has created their own constraint samplers yet, but I'm in the process of implementing one. Its hard to debug MoveIt! when plugins have no way to identify themselves into terminal, so I've added this function.

Because it modified the constraint_sampler plugin header by adding a new virtual function, it might be better to merge this into a new indigo-branch for ABI compatibility. I'm not an expert on this.

Also, there are some documentation fixes.