Add coordinateSystem property to PerspectiveLens.as

Away3D appears to default to a left-handed coordinate system, but other systems (e.g. Alternativa3D, OpenSceneGraph, etc) defaults to a right-handed projection. For compatibility with such systems I have added an option to PerspectiveLens which allows the handedness of the coordinate system to be switched.

The default state of the projection will not be affected by this change.
Merge remote branch 'ChrisDenham/patch-2'
Created enum for lens coordinate system options