Merge pull request #30 from nodiscc/master

add link: in case of empty URL (self-post), prepend "Note: " to the title
add sprites.png (toolbar icons sprite)
 * 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'
update COPYING for icons
Toolbar improvements:
 * use sprite icons in toolbar
 * remove "Home" button text (icon only)
 * make ATOM button optional, fixes https://github.com/shaarli/Shaarli/issues/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 ico
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
README: fix url/URL
README: use 200px screenshots/thumbnails
README: add tag cloud screenshot
screenshots alignment test
README: add daily screenshot
README: add tools menu screenshot