Remove the need to specify target class

The hydrator from 0.7.0 was well-received but some people complaints that it was harder to create because of the need to specify the target class at creation, hence needing to create one factory for each hydrator.

Hydrator is now smarter, and this parameter has been removed so it's simpler to instantiate a hydrator. If the same hydrator is called twice for the same object, it does not prepare again the hydrator.

If you like it, I'll update doc and test.