skip creation of unused Bluelist contenttransformer
added a servlet which can create preview images, preview tumbnails and
preview pdfs from web pages, i.e.:
http://localhost:8090/api/snapshot.png?url=http://yacy.net/en/&width=128&height=128
http://localhost:8090/api/snapshot.jpg?url=http://yacy.net/en/&width=128&height=128
http://localhost:8090/api/snapshot.pdf?url=http://yacy.net/en/

This supports also an on-the-fly generation of the preview documents if
the user is an administrator. Otherwise, the servlet fails.
To enable this, you must add wkhtmltopdf, imagemagick and (on headless
servers) xvfb to your operation system.

for detailed instructions, see
https://gitorious.org/yacy/rc1/commit/97f6089a41a4ed40aef84f692690e30f50585f5d