Add VirtualBox shared folders creation

This includes a new commandline flag to control this behavior, for those who don't want the default (which is currently automount `/Users` for OS X, and automount `C:\Users` for Windows, but will change to automount `homeDir()` at some point in the future).

The commandline flag (`--vbox-share=shareDirectory[=shareLabel]`) defines a series of essentially key-value pairs for VirtualBox shared folders in the format `shareDirectory[=shareLabel]`, where `shareLabel` becomes the in-guest mount location (and it defaults to the value of `shareDirectory`).

It also supports `--vbox-share=disable` in order to explicitly disable any automatic sharing.
Merge pull request #258 from tianon/vbox-sharing

Add VirtualBox shared folders creation