Toolbar and README improvements, make ATOM button optional...

Toolbar improvements: 

 * use sprite icons in toolbar
  * icons from http://fontawesome.io/, converted to png using http://fa2png.io/
  * Licensed under SIL Open Font License (http://scripts.sil.org/OFL)
  * sprite generated using imagemagick 'convert *.png +append sprites.png'
  * even with more icons, the new sprite PNG is 5306 bytes, whereas previous icons totaled to 5520 bytes. It also means less HTTP requests.
 * remove "Home" button text (icon only)
 * make ATOM button optional, fixes shaarli#24
  * ATOM button display is now configurable using the SHOW_ATOM variable in index.php or data/options.php (defaults to false)
 * Change "RSS Feed" to "Subscribe" for ease of use
  * Users unfamiliar with RSS will get the meaning of the button, while users familiar with it will recognize the RSS icon

Update README:

 * Add screenshots
 * Use Mediacrush as a CDN for images to reduce load on sebsauvage.net
 * Improve page layout/sections
 * Reorder list of features
 * Add links to wiki and bug tracker
 * Add proper download links for stable/dev channels
 * Improve copyright/license notice

**Work in progress, do not merge yet.** To do:

 * use sprites.png for other icons/buttons and delete old individual icon files.
 * use 200x200 thumbnails for screenshots in README for better readability/layout
 * ... ?

A live demo is available at https://telecom.dmz.se/links/  
The updated README is viewable at https://github.com/nodiscc/Shaarli/tree/toolbar-icons  
Comments are welcome on whether these changes are welcome or not.