pdf_to_imagefield 6.x-2.x-dev

This branch contains some refactoring, moving the action into the nodeapi hook.
New features are
- easy option to efficiently just render the front page of a PDF : set the target imagefield to single instead of unlimited
- Should delete (all) the derivatives when the source PDF is deleted (needs testing)
- Uses batch API (progress bar) to build derivatives immediately, not on cron.

UNSTABLE, but feel free to try it if you can also locate and patch issues