Issue #15 fixed

Further improvements were made, namely: 
    Fixed bug when some BTs were displayed incorrectly if they long enough before "from' date;
    Modified layout for GetCalendar in order for input form to take less space and validation messages to display in one row;
    Refactored code of CalendarToPdfExporter to make it cleaner;
    Added validation for dates when trying to print calendar with proper messages;
    Added method to mark today with green color on calendar's header