Add action item class, which action sheet now uses exclusively

Add methods to add items to action sheet after initialization
Add NSArray init method for action sheet, in addition to va_list init
Fix index off by one for cancel and destructive buttons

Add Doxygen comments for action sheet init methods, and a bare init method with no title or buttons
Add .gitignore modeled after github/gitignore/Objective-C.gitignore
Add demo project