Add option share folders in other direction

[rrva]

Add an option to share folders with boot2docker as NFS server and OSX as client.

This is useful for situations where the underlying filesystem needs to be a Linux filesystem.

[tonivdv]

Hey @rrva ,

This is not really part of our needs, but if you have the time to work on it and provide a PR we can certainly accept it.

[rrva]

I will try something along https://github.com/ErezHorev/dockerized_nfs_server