(Issue #30) pymaging.image.flip_left_right should not flip image vertically

pymaging.pixelarray.copy_flipped_left_right no longer transposes the
rows of an image.

Also fixed offset/index error in pymaging.pixelarray.copy_flipped_top_bottom
where the start position for the source image did not account for the
row length.

This should fix the initial problem reported in issue ojii/pymaging#30