python access to driver namespace, rather then have a textblock defined for drivers...

python access to driver namespace, rather then have a textblock defined for drivers allow scripts to add functions directly.
- bpy.app.driver_namespace, read-only dictionary, edit in-place.
- reset on file load and new file.
- on errors the namespace used to be reset, this should not be needed.

simple example.

  def driverFunc(val):
    return val * val
  bpy.app.driver_namespace['driverFunc'] = driverFunc

... now all drivers can access this function.