Support init or setup returning something other than new instance ~13

[justinbmeyer]

For canjs/canjs#721 it would be nice to make it possible for the constructor to return something other than the new instance. This is useful for singleton type setups.

[justinbmeyer]

Somehow, we'd need a way to allow setup, or some other function, to return a value other than the instance being created.

Hub.extend({
  setup: function(){
    if(IN INSTANCE STORE){
      // somehow signal to return the instance in the store instead of `this`
    }
  }
})

Setup can return an array which will be used as the arguments to init. Perhaps it can return some other flag like new Construct.ReturnValue( somethingElse ) ... which can be used as the return value from the constructor

[ilyavf]

This is fixed with #30