Checkbox radio button refactor

This PR simplifies the usage of `<material-radio-group>` and `<material-checkbox>`.

* remove the need for the nested `<input>`s by working directly with `ngModel`
* move `ngModel` to the radio group as it used to be repeated on every radio button
* proof of concept: add initial support for focus and keyboard, to show that other directives like `ng-focus` and `ng-blur` still work.

In total, the change simplifies the usage but still preserves the original angular features of radio buttons and checkboxes (E.g. `ng-change`, `ng-true-value`/`ng-false-value`, `ng-value`, `ng-blur`/`ng-focus`).

**Attention**
Don't merge the commits for the focus and keyboard support!
They are just a proof of concept, we need a general styling and concept for all components

/cc @IgorMinar

P.S.: @IgorMinar, we don't need to change `ng-blur` and `ng-focus` yet to support bubbling, as we don't have a nested `<input>` to listen to, but rather the `<material-checkbox>` and `<material-radio-button>` themselves should be focusable. 
Hi Marcy,
could we add better support for a11n in a separate PR which collects all the changes? So this PR just keeps the old functionality (the attribute was set before as well).