if observe in bindigns defines as array of attributes, onSet can return array of values for those attributes
set attribute value null if no value for the attribute in the array, which returned the method onSet
Check val is array before attrs create in onSet
Merge branch 'master' of github.com:sixdayz/backbone.stickit into set

* 'master' of github.com:sixdayz/backbone.stickit:
  Check val is array before attrs create in onSet
  set attribute value null if no value for the attribute in the array, which returned the method onSet
  if observe in bindigns defines as array of attributes, onSet can return array of values for those attributes
added set callback and some tests