v1.2.0

Fix for #2

Note: behavior of __len__() has changed: Before version 1.2.0, __len__() always returned the length of the underlying collection. Due to (undocumented) behavior of list() and tuple() which expects the __len__() method to only be available when size is known up front, __len__() will now through an error when the underlying collection does not have a known length. The len() method behavior is unchanged (i.e., it always returns the size).