Tweak styles.

* remove border from images
* use `inline-block` for the logo
* remove a couple of duplicate properties
Compress bootstrap-expo.jpg with kraken.io.

Before: 862279 bytes
After:  792647 bytes
Use https for the twitter links.
Make feed.xml valid.
Use the variable from _config.yml.
Concatenate the CSS files.
Remove unused classes.
Use the `site.url` variable in more places and avoid repetition.