x86_64 precedence changes integrated

[Gustavo Niemeyer]

Hey guys,

I've just committed a change which should make packages with color 2
(x86_64, etc) have precedence over packages with color 1, which I
believe is equivalent to the patch "smart-better-x86_64-support" I
had pending in the integration queue.

I've also added some tests on that area to ensure it continues
to work.

Please let me know if you try it out.

-- 
Gustavo Niemeyer
http://niemeyer.net