Fix KWContextNode, KWExample, KWExampleSuite deallocations after each te...

Fix for bug https://github.com/kiwi-bdd/Kiwi/issues/556.

Now, after the KWExample test has been finished, it will be removed from KWExampleSuite and deallocated.
Moreover, If there is no more KWExample's in KWExampleSuite, KWExampleSuite will be removed from KWExampleSuiteBuilder and deallocated too. 

Also I've changed additional code to break retain cycles.