Reworked SteerForNeighbors (MAJOR CHANGE)

This is a significant change, which means that:

1) I am not likely to do a point release in the near future for it, and
2) You should really read the documentation on the issue below before
   pulling this revision into your codebase if you're using any of the
   SteerForNeighbors descendants.

https://github.com/ricardojmendez/UnitySteer/issues/18