Comments, media and read position

We fixed some issues and added some new features as part of our coursework for an Open Source Software class this semester.

New features
  * Add comments to slides
  * Remember last viewed slide
  * Add YouTube/Vimeo video to slides

Improvements
  * Replaced calls to eregi with preg_match
  * internationalised hardcoded English strings
  * Fixed deleting of slideshow instances
  * Fixed display of thumbnails in caption editor