Fix memory leak in eventable __events__ hash

Based off of a current leak in rubymotion(http://hipbyte.myjetbrains.com/youtrack/issue/RM-534), I have fixed a leak in the eventable module.  Creating the hash of events as it was

```
@__events__ ||= Hash.new { |h,k| h[k] = [] }
```
will create a circular retain, and the object will never be deallocated.  Removing the proc after Hash.new resolves the issue.