Add a test for event unbinding.

Previously, I could comment out the entire off function without failing any tests.
Rename currentParentScope to currentExecutingScope
Fix potential issues with binding to __obj__ properties.
Add N=4 & N=40 versions for listeners-and-property-changes speed test
Refactor _events to improve performance with many listeners on a single property #25
Refactor scope.unbindAll and model.off to unbind by property name for efficiency #25