rework text-fields: use '&' binding instead of '@'
    
Using @ binding, we were able to manage expression to string. By replacing with a &,
we can still use expressions but don't need to manage angular templating anymore. And
we can have real boolean value.
This commit also update the demo to show more case and use real ng-model.

BREAKING: the text-fields parameter do not take a string anymore. They need boolean values.