Dropped twig documentation due to licensing issues

Sadly it is unclear under which license the twig documentaion stands. To no
longer delay twig in Debian, the documentation (for now) has been dropped
out. This results in:
- No longer building binary package twig-doc
- A +dfsg added in the version
- Repacked upstream's source to not include the documentation at all

For further information please reefer to:
https://github.com/fabpot/Twig/issues/1118
Fixed a problem with twig gets installed into the wrong directory

In Debian we have /usr/share/php in the include_path of php. The directory
/usr/share/php5 serves a completely different purpose, than containing code
written php.
Fixed a problem with twig C extension not working

...since twig.ini got not installed in /etc/php5/mods-available/
Added a README.Debian for twig
Added a README.Debian for php5-twig-extension