Added 'moment' option so that moment.js can be injected

Pikaday assumes moment.js will be a global variable. Sometimes dependencies are wrapped inside of a dependency resolution mechanism, and so normal global variables for libraries do not exist. With this update one can pass in moment.js as an option:

var moment = dependencyMechanism.get('moment');
var pk = new Pikaday({
  moment: moment
  ...
});