hideCalendarsOnRange option added; updated readme

My client found it confusing that the calendars did not appear when a "preset" range (e.g. yesterday, last 30 days, etc) was chosen.  He wanted it to work a bit more like Google Analytics' date range selector, which leaves the calendar open even when a preset is used.  To accomplish this I added the "hideCalendarsOnRange" option.  It defaults to TRUE, leaving your default behavior, but if you set it to FALSE the calendars will remain in view.