Add findOne() convenience method on Container

There are situations when you want to find a single node by name or id. I don't see any convenient way to do this in KineticJS since `Container.find()` always returns array. `findOne` convenience method can be found from DOM API to ORM libraries, so I believe KineticJS should have one too :)

I have added a `Container.findOne()` convenience method and a simple test. it returns a node or undefined.