Add nice Icon for TNTuple / TTree objects

At the moment TNTuple / TTree objects appear as a red X:
![Screen Shot 2013-03-20 at 2 24 00 PM](https://f.cloud.github.com/assets/852409/280683/af47014a-9161-11e2-8c9e-e64436f59803.png)

In the ROOT browser these have dedicated (but very low resolution) icons:
![Screen Shot 2013-03-20 at 2 25 26 PM](https://f.cloud.github.com/assets/852409/280684/b1cf1272-9161-11e2-8b80-79dd82bde86b.png)

Should we copy over the ROOT icons or get / make better ones?