Added support for Unicode code points

According to http://www.w3.org/TR/n-quads/#sec-grammar UTF-8 is a must, but Unicode code points are also allowed. Some triplestores (at least AllegroGraph) only support Unicode code points. We had to implement this fix to add support for special characters, including Chinese and Japanese. This code should be backwards compatible as the default value in the constructor is false. Unicode code points conversion can be enabled by calling `new NQuads(true)`.
This seems like overkil. Wouldn't something simple like `json_encode($str, JSON_HEX_TAG | JSON_HEX_QUOT), '"')` do the job as well?