Added support for shadow appearance selectors

Usage:

    [[C360PopoverBackgroundView appearance] setShadowColor:[UIColor blackColor]];
    [[C360PopoverBackgroundView appearance] setShadowOpacity:@1];
    [[C360PopoverBackgroundView appearance] setShadowOffset:[NSValue valueWithCGSize:CGSizeMake(10, 10)]];
    [[C360PopoverBackgroundView appearance] setShadowRadius:@10];