Removes hacky text-indent from pager and use height/padding/overflow to show images

 modified:   css/slidethis-theme.css - from
http://webavenue.com.au/best-way-to-hide-text-from-link-when-using-background-image/
Adds comments to explain optional theme styles

 modified:   css/slidethis-theme.css - explains how pager width is specific to
              the number of pager images and pager element margins
Fixes pager list bullets, which are still visible in Firefox/IceWeasel

 modified:   css/slidethis-theme.css