Python3 quicktest

Hi, PyAssimp's quicktest.py now works in Python 2/3 and runs directly
from the GIT repository directory.

The main modifications are:
  - Use new style print() syntax to ensure Python 2/3 compatibility.
  - Import PyAssimp  from parent directory inside repo.
  - Re-raise an error in core.py instead of terminating the program ( 'quicktest.py' now actually runs its course).
  - Cleanup to make quicktest.py PEP8 compatible.

Best, Oli