coloring ref. labels within history

This looks beatiful in my eyes :). Colors taken from GitX. The color for the current branch is the same everywhere in MGV now, which is a nice thing.

These colors could be customizable within the settings - but that's a topic for v0.2. For now let's take fixed colors and make a redmine ticket out of the issue :).

This is how it looks:
![mgv-history-colors](https://cloud.githubusercontent.com/assets/440517/2938179/ea9f650c-d8f5-11e3-929c-de3ac6ea3c20.png)

@claus007 FYI :smile: