Degrade more gracefully with IE7/8, fix 9/10.

- Remove PIE.htc (actually breaks IE9).
- Re-introduce some shades of gray for IE<9.
- Shield 'class' as it breaks IE7 with an exception.
- Use traditional <img> blocks for navigations arrows, as IE<11 doesn't support
  opacity for backgrounds.
Merge branch 'master' of github.com:wavexx/fgallery
Also fix the header background opacity for IE<9.