Add alpha support for all API levels

This implementation uses a Paint object to draw frames with alpha, making this possible:
![sc20140507-215028](https://cloud.githubusercontent.com/assets/2378369/2909372/b9fc0fc0-d631-11e3-9d23-c824ea412eae.png)

View's setAlpha didn't work on older devices so I had to find an alternative. I think this one is worth pull-requesting.