Added setBackground for ILoadingLayout to set background for loader

*Please be gentle - it's my first pull request and odds are that I'm not following protocol to 100%*

I wanted the LoadingLayout to be more visually distinct so I added setBackground(Drawable) so that the background of the LoadingLayout can be changed:

![image](https://f.cloud.github.com/assets/2177761/114395/1af8ee12-6b9c-11e2-8d52-aeb058556e20.png)

There is one issue with this code: the background fills the entire space below the list (in my case) but only for the first refresh. Subsequent refreshes shows the LoadingLayout pretty much as `wrap_content` and I couldn't find a way to fix this. I'm *assuming* that that's a general bug that just wasn't visible before setting a contrasting background:

![image](https://f.cloud.github.com/assets/2177761/114416/6da800de-6b9d-11e2-97e1-a0893996ac72.png)