Refactor DNS

1) Move the files below {src,test}/protocols from {src,test}/net.

2) Move classes into the ight::protocols::dns namespace.

3) Remove the leading DNS from names, i.e., DNSResponse -> Response.
Replace ight::protocols::dns::Settings with ight::common::Settings
dns.hpp: tweak example usage
Merge pull request #52 from bassosimone/refactor/dns

Refactor dns code