todayBtn, showWeek and extra markup in month view

1) Make the todayBtn option work as described in the doc (false = no
button, true = browse to today view, 'linked' = affect the input)
2) New option: showWeek that shows an extra column with the week number
3) Month View: add a "day" class and invisible" AM and PM divs in the
Day divs to be able to apply styles
minified JS