"Powered by emscripten" logo

Made a modified version of emscripten logo with "powered by" over it, and changed the one in shell.html to it.
![emscripten_powered_by_logo](https://f.cloud.github.com/assets/4681820/2236335/d57971f0-9b5a-11e3-8ef3-6ffd4e413dd8.png)