MergeableList.remove() and pop() do not remove keys from _order

These:

    def remove(self, value):
        if len(self) == 1:
            raise ValueError("MergeableList cannot be empty.")
        index = 0
        for current_value in self:
            # important! use the data in self first 'cause mergeable lists
            # can be compared with list and tuples but no the other way around
            if cmp(current_value, value) == 0:
                del self[index]
                return
            index += 1
        raise ValueError("list.remove(x): x not in list")

    def pop(self, index):
        if len(self) == 1:
            raise ValueError("MergeableList cannot be empty.")
        value = self[index]
        del self[index]
        return value

do not update self._order, which means uuids are left hanging there. This results in errors in self._get_ordered_keys(), which is used by a lot of the API...