Class: more explicit mixin configurability

Default configurable property in Object.defineProperty is false.
Going to write it out more explicitly and using ES5 syntax.
Class: enforcing extend() and mixin()

They become guaranteed methods using Object.defineProperties()
Class: rethrow catch for IE8

IE8 fails when a try block is not followed by a catch block. In order to
use the finally block, we must catch. However the catch swallows the
exception, so we rethrow.
Class: guaranteed methods are non-enumerable

The three guaranteed methods to Class subclasses: extend, mixin,
inherits, are all non-enumerable. This they can't be accidentally
touched in a bad for..in.