Rename Image.ANTIALIAS into Image.LANCZOS

When `Image.ANTIALIAS` was initially added, it was the only high-qality filter based on convolutions. It's name was supposed to say that.

Starting from Pillow 2.7 all resize method are based on convolutions. No more difference between them except filters. And filter names are "bilinear", "bicubic", "lanczos". Not "antialias", because all of them are antialias from now.

Of course we should leave `ANTIALIAS` constant for backward compatibility and make it an alias (sorry for this pun).