A Vagrant file for developing on other-than-Linux

This makes an Ubuntu-based vagrant image that has
 - docker installed
 - go tools installed
 - weave dependencies installed
 - $GOPATH set to ~
 - the local working directory mapped as a synced folder into the
   right place in $GOPATH

So one should be able to

    macosx:weave$ vagrant up
    macosx:weave$ vagrant ssh
    vagrant:~$ cd src/github.com/zettio/weave
    vagrant:weave$ make -C weaver

In addition there's a hook for local config (create a file called `Vagrant.local` in the same place), which I use to also share the weavedns repo.