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.

source/blender/python/BPY_extern.h

@@ -118,7 +118,7 @@ extern "C" {
 //	int BPY_do_spacehandlers(struct ScrArea *sa, unsigned short event,
 //		short eventValue, unsigned short space_event);
 //
-//	void BPY_pydriver_update(void);
+	void BPY_reset_driver(void);
 	float BPY_eval_driver(struct ChannelDriver *driver);
 //
 	int BPY_eval_button(struct bContext *C, const char *expr, double *value);