IE 8 specific implementations of SimpleCheckBox and SimpleRadioButton

This demonstrates a solution to the GWT compatibility issue I mentioned in #227. This solution uses a ChangeHandler for all browsers except IE 8. In IE 8 it uses a ClickHandler for SimpleCheckBox and a workaround solution for SimpleRadioButton.

This is achieved with [GWT deferred binding](http://www.gwtproject.org/doc/latest/DevGuideCodingBasicsDeferred.html).

What do you think?

If you like this I will change #227 accordingly.