VersionDict threading and Iterable in TreeDict

[chicao]

• Added the threading option to the copy in VersionDict object
• [Suggestion] Consistency between meaning for key and key_func (key in argument is clear, but when in attributes in the meaning of a KEY function, better to keep as attr key_func)
• Added the option to create a key==value TreeDict object passing an Iterable

> td = TreeDict([1,2,3,4,5])
> td
TreeDict(1=1, 2=2, 3=3, 4=4, 5=5)