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

usage ./lint path/to/file.py

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

Mostly about the changes on types hierarchy

Also do not send change to MH if deletion is not done

I put a for loop over the list to generate a foreign key for each item in the list.

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