Add an up to date Gentoo box

I'm adding an up to date Gentoo amd64 box I've just built with veewee (https://github.com/jedi4ever/veewee/pull/515).
It validates with veewee, ships with virtualbox 4.2.6 guest additions, chef and puppet.

Also, I suggest that the old `Gentoo 64` box is removed. It's at least 2 years old and updating it takes recompiling the whole system which is a major pain.

The same applies to the `Gentoo 3.3.8 i686` box: ~130 packages need to be emerged, a few /etc entries need manual update. I'm not sure if you want to keep it or not since it's a different achitecture.