Multidimensional array support fixed (#62)

The JSON Form library should now be able to support multi-dimensional
arrays, in other words "array" keys in the schema whose item is of
type "array" such as:

{
  "schema": {
    "arr": {
      "type": "array",
      "items": {
        "type": "array",
        "items": {
          "type": "string"
        }
      }
    }
  }
}

Note the "jsonform-defaults" library still needs to be updated
accordingly as "setDefaultValues" has the same limitations as
the JSON Form library.