Update buttons.js

Although with this modification, you might take a slight performance hit, it allows for dynamic data binding to the radio button values. This is especially useful when the value changes based on a $resource (or other dynamic $http related value) and a promise isn't used. Specifically, I've used this in a "page size" toggle (show 5, 10, 25, etc.), where the "show all" button has a dynamic value based on the total number of records available to the user in the database.