tesseract: Added MeanTextConf() and SetVariable methods

Added to tesseract:
MeanTextConf() to get OCR confidence;
SetVariable to set different tesseract configuration variables;

Examples:
    var res = tesseract.SetVariable('tessedit_char_whitelist', '0123456789');
    console.log('SetVariable', res);
    tesseract.image = image;
    var text = tesseract.findText('plain');
    var res = tesseract.MeanTextConf();
    console.log('MeanTextConf', res);