fix compilation issue for ARC projects
add a shorter version of the oberver method with callback which assumes common default values for options:0 and context:nil, since the callback doesn't make use of these anyways
add binding methods for automatically setting a property on an observer with the new value of a changed property on an observed object
cleanup, spelling fixes, and change usage of observee to observable (which is what apple calls it)
update public interface description comments
fix memory leak for non-ARC
add unbind to remove bindings - overrides the Cocoa version but calls the original as needed