Add Proxy Support

[tknerr]

In order to make it easier to work with Bills Kitchen behind corporate firewalls I added some scripts to set the proxy across various places:

• proxy-on will set the proxy env vars (you should edit that to fill in your proxy server)
• proxy-off will unset the proxy env vars

I also added @tmatilai's excellent vagrant-proxyconf plugin for setting the proxy inside Vagrant VMs.

The following tools should work now behind a proxy:

• gems / bundler (uses the env vars)
• git (uses the env vars)
• kitchenci and the VMs it runs (uses the env vars)
• boot2docker (detects env vars in b2d-start, i.e. needs to be restarted after proxy change)
• vagrant (uses the env vars)
• VMs running in vagrant (via vagrant-proxyconf plugin)