Extensible<T> fails on accessing static properties

When accessing static properties of T through Extensible<T> (e.g. BigInteger.Zero), a type error is raised. Accessing instance properties (e.g. String.Length) or static fields (e.g. Int32.MaxValue) works fine.

Example:

import clr, System
clr.AddReference('System.Numerics')

bi = System.Numerics.BigInteger(42)
assert bi.IsEven     # OK
assert bi.Zero == 0  # OK

class mybig(System.Numerics.BigInteger): pass

ebi = mybig(42)
assert ebi.IsEven     # OK
assert ebi.Zero == 0  # TypeError: Zero() takes no arguments (0 given)

This issue is blocking #52.

Start by running the supplied Python repro with System.Numerics.BigInteger and the derived mybig type, then trace the Extensible static-property access path. Done means static properties such as BigInteger.Zero work through Extensible without raising a TypeError, while the existing instance-property and static-field behavior remains intact.