Allow for unescaped unicode characters. Closes #4.

This includes a fallback for 5.3 to replace the unicode characters encoded by json_encode without the unescaped unicode bitmask option.
@cjsaylor `mb_convert_encoding` is part of the `mbstring` extension. Probably you need to check if it is enabled before use or add as dependency on the readme/composer for 5.3. I would prefer to make it optional and mention on the readme that limitation if you don't have the extension loaded.