Added minimal vagrantfile

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

    $ vagrant up
    $ vagrant ssh
    vagrant$ cd src/github.com/zettio/weave
    vagrant$ make -C weaver
Added hook for local vagrant config

If there's a file Vagrantfile.local, run that too. This can be used to
e.g., add the weavedns directory as a synced folder too.
Merge pull request #121 from squaremo/vagrant_dev

A Vagrant file for developing on other-than-Linux

Closes #119.