[SIMD] Add SIMD Mandelbrot sample app
[SIMD] Reorganise and clean-up

* Remove unnecessary files
* Clean up HTML a little
* Rationalise CSS
* Move main file to index.html
[SIMD] Add Crosswalk manifest
[SIMD] Scale body and display in landscape

<viewport> doesn't seem to work correctly for this example,
so I used a manual scale in JS instead.