escape JSON string correctly according to [rfc4627]

  Fixes #113

  this code escapes quotation mark, reverse solidus, and the control characters
   (U+0000 through U+001F) in the json string according to [rfc4627](http://www.ietf.org/rfc/rfc4627.txt)
  in addition, this code will escapes \u2028 (LINE SEPARATOR) and \u2029 (PARAGRAPH SEPARATOR).