Add bounding box calculation in calculations.py
Update test_bbox.py
some modifications ...
Update calculations.py
Update test_bbox.py
Update calculations.py
First test on auto_scale in view ortho or perspective. The file examples/basics/scene/test_autoscale.py is a test use middle mouse button to autofit the view.

next step :
- Travis, comment, ...
- take account of transform on visual entity
- create a pan function
Merge branch 'master' of https://github.com/sylm21/vispy
take account transformation
problems with entity_transform procedure
Merge branch 'master' of https://github.com/vispy/vispy.git
bugs fixes