Draw background image instead of color when set

This pull request allows drawing an image in the background instead of just specifying only a background color. A background image takes precedence over a background color.

The background image is not rounded by this pull request, but you may set an [RoundedDrawable](https://github.com/vinc3m1/RoundedImageView/blob/master/roundedimageview/src/main/java/com/makeramen/RoundedDrawable.java) found in the [RoundedImageView library](https://github.com/vinc3m1/RoundedImageView/).