Discourage the use of obj models

A long long time ago we [discussed](http://echelog.com/logs/browse/libgdx/1363993200) (around 19:28 for me) the meaning of ObjLoader. My opinion was that obj might be useful for beginners. And, for example, I do use it in the tutorials so it doesn't overwhelm by diving straight into fbx-conv. Moreover, obj is supported by virtually every modeling application and it is currently the only path not requiring a conversion.

That being said, it should be noted that ObjLoader is currently not fully implemented (specifically materials). The specification itself is really basic and doesn't implement all features the 3d api supports. Not all modeling applications implement the obj specification the same. And the dependency of an additional mtl file can be a bit cumbersome for some.

Unfortunately it appears that quite a few are using obj files and (naturally) experiencing problems. So I thought it might be a good idea to do something about that. I see three options (all of which with (I think) obvious pros and cons):
- Fully implement the OBJ specification
- Completely remove OBJ support (out of the core)
- Discourage the use of OBJ files

This PR tries to do the last option by logging an (optional) error every time a model is loaded. But other options/opinions are welcome.