Rewrote the _cellSize method to properly support varying screen sizes on iPhone. We take the width of the keyWindow and divide it by the number of days on the current calendar. (If the calendar view doesn't have a calendar object, we'll fall back to `[NSCalendar currentCalendar]` instead. This doesn't provide great iPad support, but we're getting there now, slowly.
Tweak cell to center label.
Implement some sort of rotation support for those who want to play with an iPad version.
Added an asset catalog.