Alignment and sons.

Trying to fix alignment I found:

 1. That the when no visible objects where present, desired acceleration
    was returning NaN. So I fixed it in Alingment and separation.
 2. visible_objects was returnin myself (as a boid). So I splitted the
    method World#visible_for into:
     a. World.visible_from(visibility conditions)
     b. World.visible_for(boid)