Fixed normalization issues within Select/Checkbox/Radio widgets

Fixes https://github.com/funkybob/formulation/issues/14

I modified the template tag to force_text the widget value and added tests for TypedChoiceField (on top of the existing one) and TypedMultipleChoiceField.

Looks like everything works as expected. I verified the Django source for the Select widget, and it's indeed also the value that's normalized before the HTML is outputted.