Implemented allowSingleDay flag, setSelectedRange(), made selectedDays chronological, simplified code

[jt-gilkeson]

• Added allowSingleDay flag which if set false won't allow the last day to be the same as the first.
• Added setSelectedRange() which allows the calendar to be initialized with a range pre-selected.
• Added DatePickerControllerAdapter so you only have to override things that you use.
• Added getCalendar to CalendarDay
• Made selectedDays first and last represent chronological date order. Removed generic type.
• Simplified logic for getDaysInMonth(), date comparisons for sameDay, prevDay
• Documented and simplified drawMonthNums logic
• Updated readme (added missing onDateRangeSelected, added allowSingleDay flag, added DatePickerControllerAdapter)

[jt-gilkeson]

Feel free to pick and choose what you want. Most of the changes are very generic but some are specific to what I needed (selectedDays change).