implement PEP 465: dedicated infix operator for matrix multiplication

http://www.python.org/dev/peps/pep-0465/

New syntax: "c = a @ b" and "a @= b"

Includes a partial backport that works with the special methods at the Python level,
but not (properly) with extension types, which lack the necessary slot methods.  Also
currently lacks the subtype special casing that Python does for the numeric special
methods.
Merge pull request #288 from scoder/pep465

Implement PEP 465: dedicated infix operator for matrix multiplication