Radio buttons and checkboxes styled for <input type="radio" /> and <input type="checkbox" />

Radio buttons and checkboxes with <input type="radio" /> and <input type="checkbox" /> styled in the same way as the ones using <button /> markup.

Tested with:
* Chrome 23
* Firefox 18
* IE 9

Example:
``` html
<div class="btn-group">
  <input type="checkbox" checked id="checkbox_1" value="1" />
  <label class="btn btn-primary" for="checkbox_1">Checkbox one</label>
  <input type="checkbox" id="checkbox_2" value="2" />
  <label class="btn btn-primary" for="checkbox_2">Checkbox two</label>
  <input type="checkbox" disabled id="checkbox_3" value="3" />
  <label class="btn btn-primary" for="checkbox_3">Disabled checkbox three</label>
</div>
```