Allow source to be initialized with an instance of an orbit source

Currently you can only initialize a source/store by specifying the class of the orbit source e.g. 

var LocalStorageStore = EO.Store.extend({
  	orbitSourceClass: OC.LocalStorageSource,
  	orbitSourceOptions: {
	  namespace: "ember-orbit-todos" // n.s. for localStorage
	}
});

This PR allows the source/store to be initialised with an instance e.g.

var schema = Schema.create();
var LocalStorageStore = EO.Store.extend({
  	orbitSource: new OCMemorySource(get(schema, "_schema"), {});
});