Minor changes on size() and addition of isEmpty()
Minor changes on size() and addition of isEmpty()

img.size() led to an error if img was empty. Now, it returns (0, 0), checking
width and height before calling cv.GetSize. This also adds img.isEmpty(), which
checks if size equals (0, 0).

Fixes #124

beni55/SimpleCV,nils-werner/SimpleCV,jlegendary/SimpleCV,jyt109/SimpleCV,nils-werner/SimpleCV,hayd/SimpleCV,sightmachine/SimpleCV,nils-werner/SimpleCV,tpltnt/SimpleCV,gskielian/SimpleCV,sightmachine/SimpleCV2,jayrambhia/SimpleCV2,gskielian/SimpleCV,tpltnt/SimpleCV,jyt109/SimpleCV,jyt109/SimpleCV,hayd/SimpleCV,hayd/SimpleCV,sightmachine/SimpleCV,hurdlea/SimpleCV,jlegendary/SimpleCV,beni55/SimpleCV,onaclovtech/SimpleCV,onaclovtech/SimpleCV,onaclovtech/SimpleCV,jayrambhia/SimpleCV2,sightmachine/SimpleCV2,sightmachine/SimpleCV,gskielian/SimpleCV,beni55/SimpleCV,tpltnt/SimpleCV,beni55/SimpleCV,jayrambhia/SimpleCV2,gskielian/SimpleCV,hurdlea/SimpleCV,onaclovtech/SimpleCV,jyt109/SimpleCV,onaclovtech/SimpleCV,jlegendary/SimpleCV,hayd/SimpleCV,jayrambhia/SimpleCV2,jyt109/SimpleCV,hurdlea/SimpleCV,jlegendary/SimpleCV,sightmachine/SimpleCV,nils-werner/SimpleCV,sightmachine/SimpleCV2,jlegendary/SimpleCV,tpltnt/SimpleCV,nils-werner/SimpleCV,hurdlea/SimpleCV,hurdlea/SimpleCV,hayd/SimpleCV,tpltnt/SimpleCV,sightmachine/SimpleCV,beni55/SimpleCV,sightmachine/SimpleCV2,gskielian/SimpleCV