Removed dependency on proprietary GIFStreamMetadata

When compiling and running in Java 7 (which I did just to see if it worked), a warning is shown about com.sun.imageio.plugins.gif.GIFStreamMetadata being proprietary and therefore deprecated. In a OSGi context, an external dependency was required for Suns ImageIO implementation, which I could not find in Maven Central. The changes in this commit should fix this.