Add BOM support

[drizt]

Atm QJSON returns error for UTF with BOM encoded strings. QJsonDocument from Qt5 can handle with it. Qt5 has internal function

void Parser::eatBOM()

{
    // eat UTF-8 byte order mark
    uchar utf8bom[3] = { 0xef, 0xbb, 0xbf };
    if (end - json > 3 &&
        (uchar)json[0] == utf8bom[0] &&
        (uchar)json[1] == utf8bom[1] &&
        (uchar)json[2] == utf8bom[2])
        json += 3;
}

So JSON is good for Qt5 can be bad for QJSON.