Move $nameserver from ExecutorInterface to UdpTransportExecutor

[clue]

This changeset removes the $nameserver argument from the ExecutorInterface and adds it as a constructor attribute to the UdpTransportExecutor. This allows for a significant clean up in our APIs as most of the existing implementations of ExecutorInterface do not care about the nameserver and simply pass it through to some underlying executor. This also allows for future optimizations (such as reusing a single socket for multiple lookups) and helps with future TCP/IP executors (#19) and also multiple DNS servers by implementing fallback strategies as a hierarchy of executors (#6). These APIs are somewhat internal and it's unlikely this will affect many consumers of this package.

// old
$executor = new UdpTransportExecutor($loop);
$promise = $executor->query($nameserver, $query);

// new
$executor = new UdpTransportExecutor($nameserver, $loop);
$promise = $executor->query($query);

Additionally, we can now validate the nameserver address and throw an InvalidArgumentException if it is invalid.

Refs #131
Refs #128