Now lefactory populate a LeObject attribute named _child_classes

	The UniqID datahandler has to call the datasource
	LeObject has to be able to determine wich parent class define its UID datahandler
	[NOT IMPLEMENTED] the datasource should be able to select with an abstract LeObject as target ( see #92 )

- datahandler base class was not supporting default values due to a bug
- implements the datetime datahandler construct_data method

- Replaced a self. by cls. in leobject
- Changed default value for runtest option

- Changed the way of instanciating results object in get (using __new__ )

- now we have a LodelHook to trigger migration handlers loading
- datasources initialization in dyncode is now trigger by a hook (lodel2_plugin_loaded)
- datasources loading by dyncode is better

- The LeUpdateQuery implementation is broken or at least have huge performance issue
- Moved LeObject exceptions and LeQuery exceptions in lodel.leapi.exceptions

Adding a field to all non abstract classes to store in DB the name of corresponding LeObject subclass