Improve console output and error handling.

datepicker.setSelectedDate();
>> Error: You must provide a new date to select!

datepicker.setSelectedDate(new Date());
>> Mon Jul 01 2013 05:09:37 GMT-0300 (UYT)

datepicker.incrementMonth(2);
>> Sun Sep 01 2013 05:09:37 GMT-0300 (UYT)

datepicker.incrementMonth();
>> Tue Oct 01 2013 05:09:37 GMT-0300 (UYT)

datepicker.build();
>> <div id="date"></div>