The 'group' property on the scope used by the 'Post to' button
was set when the component was instantiated but not updated
if the group subsequently changed.

Fix this and simplify the code by using the 'bindToController'
facility from Angular 1.3.x to avoid copying properties
from the directive's scope to the controller.

This means that 'vm.group' expressions in the controller now
update when the corresponding attribute changes.