Accept 0,0 as synthesized clientX,Y coordinates

The check meant that passing 0 in as coordinate component would trigger
calculating the actual offset from the target element.
Rename as testMontage as testTarget for clarity

The intent being that the Target object used as the prototype has come
from the test environment, not the test suite environment.

This used to actually be a Montage object, not a Target.
Add specs for activeTarget concept

Always a worthwhile endeavor as I figured out I wasn't actually using
nextTarget to find the next target if the original didn't acceptFocus.

Anyway, now we have tests built on top of the hands-on playground so 
there should be enough to get a feel for how this system works.