sp new feature: compare pdf files

Requires ImageMagick for "convert" and "compare".
First basic version. Just reports "good" or "bad".
sp: compare pdf files - report page numbers
Bugfix sp compare for one-paged PDF

* one paged pdf don't follow the source-*.png
  naming scheme
* Add two sample qa files
* Add rake task for qa
PDF comparison / qa documented
First shot at even/odd table header

We now have the ability to restrict the table
head to "only on odd pages" / "..even.."

This is only the first attempt, lots of things are
still missing.