Proxy guest DNS requests through the host machine

Rather than attempting to handle DNS lookups itself, this causes
the virtual machine to proxy any DNS back through the host machine.

This should really help in cases where your DNS changes due to an
unfamiliar network, etc... We'll want to pay attention to how it
handles local host lookups, though I think the hosts file will
still be watched.

See #327
Merge pull request #347 from Varying-Vagrant-Vagrants/enhance/natdns

[PR] Proxy guest DNS requests through the host machine