Check if class exists before trying to instantiate

Currently balanced-ruby will [parse the uri](https://github.com/balanced/balanced-ruby/blob/master/lib/balanced.rb#L50) and try to load a class by name even if it does not exist. 

We need to change this behavior so that it logs a warning (e.g. `Resource <class_name> does not exist, you should upgrade to a newer version of balanced-ruby`) and does not explode in a flaming mess.
