Feature/nfs

NFS Synced folder implementation is complete. It requires at least one `:private_network` interface with `:static` type of IP detection. For example, it should to work fine with both of Linux and OS X guests:
```ruby
Vagrant.configure("2") do |config|
  config.vm.network :private_network, ip: "192.168.0.19", network: "255.255.0.0"
  config.vm.synced_folder "./host_dir", "/guest_mount_path", nfs: true
end
```  

P.s. As [Vagrant NFS documentation](http://docs.vagrantup.com/v2/synced-folders/nfs.html) says, you may be prompted for administrative privileges to export NFS folders on your host machine. It means that your host's MacOS account should be permitted to execute 'sudo'.