Fixed loss of important patch/diff aspects and too aggressive diff coloring.

I like the "new" diff output a lot.  However, I think we've gone a little bit too crazy with the colors — primarily the very aggressive background colors, but also the old/new font colors.

At the same time, diff context lines were changed to appear in almost invisible light gray — even though the context is of utmost importance in patch reviews.

Lastly, Dreditor already does its best to discover code comments, and it colorized them previously — doing so gives a lot more visual structure to the presented lines of code, and on top, it _encourages_ developers to (1) write comments and (2) write _good_ comments.  Especially if you're reviewing tons of patches per day/week, this visualization is key for productivity.

Changes in this PR:

- Contextual diff lines (preceding/following lines of changed lines) are crucial, must not visually appear as unimportant, greyed-out content.
- Restore colorization of comments in added lines for readability and to encourage code comments.
- Generally decrease background color saturation to shift focus on code, instead of colors.

Screenie:

![dreditor-diff-coloring](https://f.cloud.github.com/assets/41992/1475861/1ccf2d60-4642-11e3-874b-c8f1b5f0ba14.png)