feat(buttons): added checkbox and radio buttons

I have created directives for checkbox and radio buttons respectively. I have a few things to discuss:

1) Should I create a directive which can toggle on click a custom name? (Semantic-UI has an example as the first button. When a button has a name once its cliked name changes) - as I guess I will need to use transclude
2) is the naming style of directives good?
3) Not so sure about docs style. I saw yours as a guide. Is that alright?

Hope it would be fine!
There's a new instance of the controller for every instance of the directive, it's impossible to really configure the options this way.

In angular core we have `ng-model-options` which IIRc is a directive with its own controller, so that it's possible to actually configure and share the configuration across multiple directives (due to getting it with `require: "^?ngModelOptions"`) --- so if you want to use a controller, it should be possible to do it that way too