(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.
Merge pull request #32 from cgroner/issue30

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