Add cache "bucket" for curl/wget

[nikolauskrismer]

Would it be possible to cache downloads performed via curl (and/or wget)?

[fgrehm]

Well, for now it's basically a matter of downloading files into /tmp/vagrant-cache on the guest VM. Eventually we'll get to GH-4 and you'll be able to have a better control of cache dirs.

I'm closing this as there's nothing much we can do for now but feel free to ask questions if you have them :)