Don't unnecessarily copy the key list

Conflicts:
	doc/contributors.rst
	test/test_son.py
Allow destructive ops during son iteration
and let python handle (identical to iter on list)
PYTHON-703 Remove slow SON.__contains__ method.

__setitem__ must now check self.__keys to avoid setting keys twice during
"copy.copy(son_obj)".
PYTHON-703 Fix tests for Python 2.4.