Add support for Object#method to the Eventable module

Hello!
I make heavy use of the Eventable module, and sometimes I find it useful to re-use some methods as events. It looks similar to this:

```ruby
object.on :my_event, &method(:my_method)
```

However, Method#to_proc returns a different object each time, and this makes it difficult to unbind the event when the VC gets deallocated. This is easily solved by passing a simple Method object

```ruby
object.on :my_event, method(:my_method)
```

My implementation is backwards compatible with the existing one so it shouldn't cause any problem to existing apps.

Also, be aware that the current test suite fails, as Object#method leaks. This bug has been reported and will be fixed in the next RM release (http://hipbyte.myjetbrains.com/youtrack/issue/RM-456).