refactor(checkbox): remove nested `<input>`

Closes #121

Don't use nested `<input>`:
* simplifies the usage of checkboxes
* we already render the buttons ourselves with canvas
* polymer radio component also does not use an <input> internally

Don't create scopes for the buttons
* helps preventing newbie problems with `ng-model`

Reuse original `inputDirective`
* saves code while preserving features (E.g. ng-true-value`)
* only a little bit hacky but works nice
refactor(radio): remove nested `<input>`, put `ng-model` to group

Closes #121

Don't use nested `<input>`. This is ok as:
* simplifies the usage of radio buttons
* we already render the buttons ourselves with canvas
* polymer radio component also does not use an <input> internally

Place `ng-model` on `<material-radio-group>`
* simplifies the usage of radio buttons
* before it needed to be repeated on every radio button

Don't create scopes for the buttons
* helps preventing newbie problems with `ng-model`
docs(): promote LAYOUT, remove UTILITIES

Closes #129