Ogre binary format support

**My Ogre binary importer work is now complete**

* Support for Ogre binary .mesh and .skeleton files.
* Rewritten OgreXmlSerializer to cleanup code (compared to my first pull request), new performance optimizations, bug fixes and more robust (supports other than the previously hardcoded XML node order).
* New test assets in `test/models-nonbsd/Ogre/OgreSDK` with licence and readme for furher information. Assets taken from the official Ogre3D SDK. Removed old now unnecessary test assets.

**In addition to the Ogre implementation**

* The regression tests seemed to be 2-3 years old at this point in time. I bugs/fixed issues in the `run.py` and regenerated the `db.zip` database. Success rate went from 40% to 99% in doing this. The database seemed to have lots of old files and just out of date data hashes.
* Updated `.gitignore` to ignore many kinds of files if build is performed at the root, instead of `/build`. At the moment the regression tests `run.py` assumes a root build and finds the assimp executable from `<repo>/bin`.

Hopefully this all looks good to you. The Ogre XML supports is on par with the previous implementation. I kept some "ugly" code from the old implementation so that code out there depending on the quirks wont break when updating to 3.1.

Comments and feedback is welcome!