Adjustments > Auto > Levels: now implemented

I've also removed Auto Tone as a menu item, because from what I can see,
it's extremely similar to Auto Levels (probably why Adobe swapped in
Tone for Levels in the current Auto section of Photoshop's menu).

@audioglider mentioned having Auto Color and Auto Contrast algorithms
available; I'll wait to touch those until he's had a chance to push his
versions!