Checks for EmField :
- checks that the name is uniq in an EmClass
- checks that a relation EmClass <- rel2type -> EmType is uniq

An old string was given as argument to DummyMigrationHandler and was interpreted as debug = True

Associated tests has been added/updated

Now on load default fields for EmClass are created. The EmClass.check method create them if they are not present.

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

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

On create_component on delete_component on set_backend and on migrate_handler

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

=> a type can have multiple superiors (only instance of LeObject can have one)