Change Request for InvokerDefault::_callObserverMethod()

Instead of `method_exists()` it should be `is_callable()` within an OO environment.

It should be possible that you can use observer objects with the magic __call implementation and a "non-existing" method. 

There is a nice blog post about those two functions: [http://blog.jmfeurprier.com/2010/01/03/method_exists-vs-is_callable/](http://blog.jmfeurprier.com/2010/01/03/method_exists-vs-is_callable/)

`method_exists()`: There are also 25 more occurrences in the `app` folder and 23 in `lib/Magento`. Maybe you can consider changing them also to `is_callable()`, if not it would be at least nice if you accept that PR :-)

Thanks!