Rename binary package 'twig' to 'php-twig'

As suggested by David Prévot
Add build-dependency to re2c
Make use of dh_phpcomposer
Update standards to 3.9.5 (no changes needed)
New upstream release (v1.15.1)
Synced d/copyright (the main part) with /LICENSE file from upstream
Drop out the part regarding ext/twig from d/copyright

It looks like the php5 twig C extension is now covered under the
same license, as twig itself.
Update copyright for the Debian packaging itself