Optimize default images

Optimizers used:
- PNGOut: http://advsys.net/ken/utils.htm
- DeflOpt: http://www.walbeehm.com/download/
- Defluff: http://encode.ru/threads/1214-defluff-a-deflate-huffman-optimizer
Reorganize index.html

- Set page title and subheading
- Reindent: use tabs; <head> and <body> tags have depth 0
- Remove link to GitHub
- Remove empty lines after <body> and before </body>
- Remove reference to  (inexistant) CSS for media="print"
Rename subdirs to shorter names

- images => img
- stylesheets => css
- javascripts => js