Favicon changed to logo + SVG version of logo

This should fix issue #208. 
I've reconstructed the tangram logo with the varying spacing. I think it's a better looking logo if the pieces can actually fit together, as opposed to the images I showed in the issue. 
The favicon [ICO file](https://en.wikipedia.org/wiki/ICO_%28file_format%29) contains image sizes 16, 32, 48, 64, 128, 256. Perhaps the HTML should include the HTML5 `<link rel="shortcut icon" size="16x16, 32x32, 48x48, 64x64, 128x128, 256x256" href="http://elm-lang.org/favicon.ico" />` as mentioned on [wikipedia](https://en.wikipedia.org/wiki/Favicon) and in this [thorough tutorial (that also shows how to do image tiles for mobile devices and Windows 8)](https://scotch.io/tutorials/all-favicons-for-all-devices-and-sizes).