Move social network links to header, other small improvements

+ Losslessly optimize images by about 0.4%
+ Better URL colors derived from http://ilyabirman.net
- Drop files that are no longer needed
- Drop "footer a" CSS rule

This commit was inspired by latest Ilya Birman's post (in Russian):
http://ilyabirman.ru/meanwhile/all/ne-zablednyay/