Pull request to extend the functionality

[eungkyu]

• Decoding scalars to scalars dictionary (eg. [String: String])
  Sometimes, just key-value pair is included in json
• Encode primitive type, eg. "hello, there".toJSONString()
  JSONCompatible is JSONEncodable but toJSONString() is failed.
• NSNull handling
  NSJSONSerialization.JSONObjectWithData may include NSNull, which is not handled property

[matthewcheok]

Thanks!