colors more friendly for colorblind users and still good for others

Changed BranchBehindForegroundColor = [ConsoleColor]::Blue and DarkRed to Red to make colors more friendly for colorblind users.

Especially helpful when combined with:

$Host.UI.RawUI.ForegroundColor = [ConsoleColor]::Gray
$Host.UI.RawUI.BackgroundColor = [ConsoleColor]::Black

![image](https://f.cloud.github.com/assets/553630/1873320/79ae4ee2-78b7-11e3-8058-374174b61b5e.png)