Adjust & pngout d-logo-sketch.png

d-logo-sketch.png and d-logo-sketch-small.png were previously out of
alignment, causing the stylized D to shift left when the two logos
swapped.

The behavior can be seen in this gif (watch the "Discourse" logo at the top):

![behavior gif](https://cloud.githubusercontent.com/assets/1084381/4025216/73d17512-2bef-11e4-8edb-3b16add5f5cd.gif)


I adjusted d-logo-sketch.png to be perfectly aligned with d-logo-sketch-small.png using [paint.net](http://www.getpaint.net/), then recompressed it using pngout.