[pyassimp] Simplified and beautify simple_opengl_viewer.py

Since the new SDL based viewer is much better suited to interactive exploration, simplify
the basic OpenGL viewer to make it an easy introductive read.
[pyassimp] Minor fixes to the simplified open_gl viwer
[pyassimp] Make it easier to import pyassimp

'import pyassimp' (instead of 'from pyassimp import core') is now sufficient
[pyassimp] Make sure sample.py and quicktest.py work
[pyassimp] Renamed the 3d for better clarity + added a short README
[pyassimp] Minor tuning in 3d_viewer.py
Merge pull request #22 from severin-lemaignan/master

Polishing of pyassimp code