Remove uses of __defineSetter__ and __defineGetter__.

In IE10 - and possibly older, untested - Iugo throws "Object doesn't
support property or method '__defineSetter__' when in use" when in use
and when running the tests.

This changes it to use Object.defineProperty instead.
Merge pull request #7 from bruce-one/master

__defineSetter__ is deprecated.

Thanks! Good catch.