Checking that internal='object' is reserved for common fields

Added default fields for EmClass, they are listed by classtype in EmClassType
Fixing some tests (testing that a new EmClass has no fieldgroups)
Fixing the delete_check of EmClass. Now an EmClass can be deleted if it contains only default fieldgroup and default fields

LeFactory.generatePython() is a method that generate LeObject code and python classes for each EmClass and EmType in an EditorialModel

Add a check for the default value
Removed the _fieldtype_instance attribute

Now fieldtypes are not derivated from EmField anymore

Creating a class RandomEm in EditorialModel/randomem.py

Every backend and MH are now subclasses of dummy ones. This allow to check the type with issubclass.

Make most of the model tests pass (except the tests about the backend calls and set)

On create_component on delete_component on set_backend and on migrate_handler

When the list is empty new_rank is > get_max_rank, because get_max_rank returns -1

Now modify_rank don't raise anymore when too big/small modifier is given but do its best to make a valid modification
The modify_rank tests (and the __hash__ __cmp__ tests) has been modified to fit this new behavior