Updates serializeModel to override and not call SubSerializers with a method

Allows for a method to override the call to a SubSerializer class instance
Updates Tests to reflect this change

Signed-off-by: Justin Yost <justin@loadsys.com>
Updated readme with information on methods overriding sub-serializing

Signed-off-by: Justin Yost <justin@loadsys.com>
Merge pull request #21 from loadsys/f/methods-override-for-serializing

F/methods override for serializing