Collada: tristrip support

We needed to import some COLLADA files created from a CAD exporter (which shall remain unnamed, in order to protect the innocent) which contained tristrips. As we had no way to dissuade the exporter from writing tristrips, we decided to implement tristrips in assimp's COLLADA parser.

The commits in this pull request were created as a pair programming effort, so I left myself in as Committer and my colleague as Author. If you have any better Ideas for correct attribution let me know, I'll gladly amend.
I know this was there before, but where does 20 come from?