Update html5 boilerplate to v4.3.0.
Many css related fixes.

* remove units from 0
* remove duplicate properties
* merge duplicate selectors
* remove incompatible property `display: table-cell`
* add missing vendor prefixed properties and move the unprefixed property after the vendor ones
* use the shorthand properties when possible and be more explicit with the background-color and list-style-type property
* use the (shortened) hex colors when possible and be consistent with the case used
* move overrides after bootstrap
* move inline style to style.css
* use the bulletproof syntax for fonts
Trim trailing spaces and use spaces consistently.
Remove the unused "full-screenshot.png" and "images/github-gov-black.png".
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.
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`.