Full control over handler

[xcambar]

refs #156 (comment)

(Most) mock classes expose a handler method, but currently, this method is bound to mockjax during instanciation.

The call to this method should be wrapped in order to allow the developpers to have access to all the abilities of mockjax, in rare case when the exposed API of FactoryGuy is not enough.

[danielspaniel]

the handler is now available with version 2.3.0
when you get a mock with

   import { mockFind } from 'ember-data-factory-guy';
   let mock = mockFind('user');
   mock.handler //=> the handler function used by mockjax