adding back button evaluation so you can do 1/60, 90*0.1 etc as well as dimension conversion 1km-10cm+4ft

Note...
- Python3.1 you don't need to add the .0 for divisions anymore (was esp annoying for button eval)
- Simple dimension input, imperial mi/yd/ft/in, metric km/m/cm/mm,  Later could display these values and have a pref for scene scale, atm it assumes 1BU == 1m.

source/blender/python/BPY_extern.h

@@ -122,7 +122,7 @@ extern "C" {
 	void BPY_pydriver_update(void);
 	float BPY_pydriver_eval(struct ChannelDriver *driver);
 
-	int BPY_button_eval(char *expr, double *value);
+	int BPY_button_eval(struct bContext *C, char *expr, double *value);
 
 /* format importer hook */
 	int BPY_call_importloader( char *name );