feat(buttons): provide wgCheckbox

Following the discussion started there #18.

- Added support for bootstrap3 default markup that is now pushing for label > input[type="checkbox"], cleanly leveraging core input directives.

- Reduced the scope to a single directive, that can be used to build a single checkbox or several checkboxes (ui-style).

- Renamed to `wgCheckbox` (close to ui-style)

- It is fully tested with type checking (boolean/integers/strings) with a core-like approach (@-style interpolation with type checking on link).