UTF-16 encoded comments

[Carrie Lui]

facebook return the comments and reply messages in UTF-16, but the package seems encoding in UTF-8, this cause problems for emoji display

[Jeffrey Shen]

This is because the messages are returned as javascript escaped strings e.g "\uD83D\uDE0E" which R doesn't support. This needs to be converted when parsing the json, which rjson doesn't do.

I tested this out with jsonlite and it parses the json correctly, so the json library should be changed to support this