AssetManager: Add filetype dependent loaders.

Use it like this:
```java
assetManager.setLoader(StillModel.class, ".obj", MyObJLoader);
assetManager.setLoader(StillModel.class, MyDefaultLoader);
assetManager.setLoader(StillModel.class, ".xml", MyXMLLoader);
assetManager.setLoader(StillModel.class, ".mesh.xml", MyOgreLoader);
```