feat(buttons): provide wgRadio

Following the discussion started there #18.

- Added support for bootstrap3 default markup that is now pushing for `label > input[type="radio"]`, thus cleanly leveraging core input directives.

- Renamed to `wgRadio` (close to ui-style), with project prefix `wg-*`.

- Leverages the default HTML5 `value` attribute with @-interpolation

- It is fully tested with type checking (boolean/integers/strings) with a core-like approach (@-style interpolation with type checking on link).

PS: There is console logging in the source code, will obviously be dropped before merge.

Working plunker if you want to play with it: http://plnkr.co/edit/BMSEPMZbtmrJAmSLohT2?p=preview (radios can/should be hidden with `data-toggle="buttons"` on the `div.btn-group` element.