Refactor + Preload backgrounds

Close #6
Comments
Update ref in README
Add clock.
Merge pull request #9 from Spy-Seth/feature/clock

Add clock.
Close #5
Better clock position.
Add periodic background updates
Merge pull request #14 from madx/gh-pages

Add periodic background updates
BEMify clock + .PrettyNewTab-corner(--*) class
Prettier time
Merge pull request #16 from putaindecode/prettier-time

Prettier time
Better defaults
Refactoring + footer

This commit includes
- better text shadow on clock
- better font size ?
- safer interval update for the clock
- only image is hidden during transition (close #15)
- preload is not brutal as it was: images are loaded one by one
- flicker fixes on chrome/safari
- footer with credits & links to project (close #10)
Add credits

Close #8
Add a pretty radial overlay
Add underline to links on hover
Merge pull request #17 from madx/gh-pages

Add a pretty radial overlay
Merge pull request #19 from madx/underlined-links

Add underline to links on hover
Change background loop from 15s to 30s.
Add new image.