feat(MdCheckbox): add object to possible checkbox value (#1556)

* Add object & array to possible checkbox values

* Update MdCheckboxMixin.js

Author: jkosonen

src/components/MdCheckbox/MdCheckboxMixin.js

@@ -5,9 +5,9 @@ export default {
     MdRipple
   },
   props: {
-    model: [String, Number, Boolean, Array],
+    model: [String, Boolean, Object, Number, Array],
     value: {
-      type: [String, Number, Boolean],
+      type: [String, Boolean, Object, Number],
       default: 'on'
     },
     name: [String, Number],