Losslessly compress images.

Also, add the remaining images to the repository after compressing them losslessly and replace the current "screenshot.png" with the image that's actually used atm.
Fix validation errors.

All validation errors are fixed except for the `http-equiv="X-UA-Compatible"` meta tag which is known to not be HTML5 valid but is needed in order to force usage of the latest IE.
Remove the unused Google fonts.
Update readme.md.

* fix typos
* fix wrong level of indentation
* use a code block for the local commands
Update .gitignore and _config.yml.
Remove the unused jquery.min.js.
Use the https links to the github projects for consistency.
Remove stray space after `alt=`.
index.html: remove unneeded class.

It's inherited from `ul`.