Prevent pollution of global namespace. All controls are stored under Control.contorls. For convenience, a string can be passed as "extend" option.
register flag added. The control is not registered as a global handlebars helper without register:true.
added control.helper() function to make something like …

Template.myTemplate.helpers({
  myForm: myControl.helper()
});

… possible. That way specialized Controls can be easterly encapsulated and are not globally available.