Remove unused images and losslessly compress the rest.

Also, add forkme_left_red.png in the repo and optimize it.
Add alt text and image dimensions for download-button.png.
Remove type="text/javascript".

This is no longer needed for HTML5.
Remove trailing spaces and extra newlines.
Fix a few JSHint warnings.
Include less.js after all the css files.
Use https for the Github links.
Remove the unused Twitter script.
Remove unit from zero.
templates/layout.rhtml: Minor html5 fixes.
Include html5shiv.js for older browsers.