lavc: add teletext decoder using libzvbi

Based on a patch by Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>.
http://lists.ffmpeg.org/pipermail/ffmpeg-devel/2012-December/136677.html

The original patch was rebased by Tudor SUCIU <tudor.suciu@gmail.com>.

Lots of additional features and fixes are made by me.

Fixes ticket #2086.

Changes since last version:
- change default page to all pages

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>