Apt cacher bucket

Caching for apt-cacher-ng, works only with NFS-folders, but it is still useful! E.g. while working with `docker` inside VirtualBox, I want the least intrusive way to let `docker` LXC-containers reuse packages, without sharing folders or any `vagrant`-specific plugins/settings. `apt-cacher-ng` enables it! :) it's rather cool to install packages in your containers without internet connection. 

Please consider merging it into `vagrant-cachier`, i tried to follow the code conventions. 

Cheers!