Add responsive to the API

[oyvindhermansen]

For example give the heightify a method that turns heightify off on mobile devices.
(You would need heightify on mobile mostly because the cards is just stacking on top of each other.

some pseudo code to illustrate

/**
@returns {boolean} true/false
*/
responsive() {
  if (opts.responsive) {
    // run heightify through a check
   // for when on mobile devices
  }
}