Simple validation code refactored in 'meetings.js' file

1) Variables 'mac', 'date', 'startHour', 'endHour' and 'title' in function 'addMeeting' combined  into a JavaScrip object 'meeting';
2) Added 'validateMeeting' function instead of validation in 'if .. else' blocks;
3) Added 'appendIf' function;