Added icons and added TransitioningContentControl transition feature

The following icons were added:
![icons light](https://f.cloud.github.com/assets/6247359/1831549/ec052bb0-7393-11e3-9f5f-6c72539b6372.png)
![icons dark](https://f.cloud.github.com/assets/6247359/1831550/f05a862e-7393-11e3-8d05-03fdc723936d.png)
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I had multiple TransitioningContentControls in one scope. They all needed a custom transition, but since a custom transition has to be named "CustomTransition", there was a conflict. I have added a property to the TransitioningContentControl with which you can set a custom name for the custom transition.
I have also added this feature to the Metro demo.