Add UIAppearance support for custom images.

This allows customization of the default status images used by the
`+showSuccessWithStatus:` and `+showErrorWithStatus:` class methods:

    [[SVProgressHUD appearance] setHudSuccessImage:successImage];
    [[SVProgressHUD appearance] setHudErrorImage:errorImage];
Merge pull request #160 from jparise/image-appearance-support

Add UIAppearance support for custom images.