Add assimp library Loader plugin

[kochol]

Add assimp library Loader plugin to engine to load meshes.
http://assimp.sourceforge.net/

[kochol]

Assimp library seems cant load many file formats correctly but it must be good for static mesh loading.

[kochol]

There is a new scene format with exporters for 3dmax, maya & blender. http://opengex.org/
It has many better features than collada and other formats.