Improve checked/unchecked binders for radio box

Hi there,

I'm submitting this pull request because I believe that the checked and unchecked binders should be improved.

* __Improve comparison code__  
  Currently, the algorithm to compare the radio input value to the data model value does a fully typed comparison (in js)
    e.g. `el.checked = el.value === value`  
  Considering that radio input values are always strings, this won't work with when the model value is a boolean.  
  So my suggested solution is to cast the model value to string by default.

  ```coffeescript
      checked:
        publishes: true
        bind: (el) ->
          @currentListener = bindEvent el, 'change', @publish
        unbind: (el) ->
          unbindEvent el, 'change', @currentListener
        routine: (el, value) ->
          if el.type is 'radio'
            # it was: el.checked = el.value is value
            el.checked = el.value?.toString() is value?.toString()
          else
            el.checked = !!value

      unchecked:
        publishes: true
        bind: (el) ->
          @currentListener = bindEvent el, 'change', @publish
        unbind: (el) ->
          unbindEvent el, 'change', @currentListener
        routine: (el, value) ->
          if el.type is 'radio'
            # it was: el.checked = el.value isnt value
            el.checked = el.value?.toString() isnt value?.toString()
          else
            el.checked = !value
  ```
  Note that I've only changed this code for radio inputs.
Based on that, for the "checked" binder, a radio box will be checked only if the input value is equal to the model value (after coercing it to a string).

* __Improve jasmine test case__  
  The jasmine test cases for the checked/unchecked binders were incorrectly using a DIV element as test target so the results were not relevant. 
  So I've changed them using radio input boxes and checkboxes.

I hope this helps.

Feel free to let me know if there's any issues.

Regards,

David