Mirage DNS resolver support and more examples

• Add Dns_resolver_mirage module for making stub resolution requests
  using the Mirage module types.
• Dns.Resolvconf parses /etc/resolv.conf entries using Ipaddr.t
  instead of string values now.
• Adapt Dns_resolver and Dns_resolver_unix to use Ipaddr.t more.
• Improve mldig to use Ipaddr more and add more RR printing to
  match the conventional dig tool behaviour.
• Expose Dns.Packet.Not_implemented exception rather than a pattern
  match failure.
• Depend on external Base64 package instead of bundling one inside
  the Dns packed module.
• Add a local opam file for easier pinning.
• Add an examples/ directory with a DNS forwarder sample (#21).

Fix file descriptor leak in resolver

Fix file descriptor leak in resolver (#15, #16) by expanding commfn with a cleanup function.

Fixes for Xen/Mirage backend

• Ensure that all Dns.Buf.t buffers are page-aligned, via Io_page.
• Remove a Unix dependency that snuck into the Dns_resolver portable core, by adding a timeout argument to the commfn type.
• Improve ocamldoc in Dns_resolver_unix.

Support multiple zonefiles per server

• Add process_of_zonebufs to handle multiple zone files.
• Adapt Dns_server_unix to expose multiple zonebuf functions.

IPv6 support and better portability

• Use Ipaddr.V6 to restore IPv6/AAAA RR support.
• process_query now takes an output buffer so it doesn't have to
  overwrite the input buffer it just parsed.
• Add Travis continuous integration scripts.
• Regenerate with OASIS 0.4.1
• Split the dns.lwt into a portable dns.lwt-core that doesn't
  require Unix (from which a Mirage version can be built). The only
  change to existing applications is that Unix-specific functions
  have shifted into Dns_resolver_unix or Dns_server_unix, with
  the module types for PROCESSOR and CLIENT unchanged.

Use Ipaddr and functional processor interfaces

• Add path argument to `Resolv_conf in Dns_resolver.config.
• Dns_resolver.t is now a record type rather than a first-class module.
• Fix mldig server and port options.
• Change Zone.load_zone to Zone.load and make it functional over Loader.db.
• Use Ipaddr.V4.t addresses in favor of Cstruct or Uri_IP representations.
• Fix RRSIG signed type to be of the answer rather than the question.
• Fix ANY queries.
• Add Buf to provide a nickname for char Bigarrays.
• Change Packet.{parse,marshal} to use Buf.t rather than exposing Cstruct.t
• Change Packet.parse to remove name map parameter
• Factor protocol modules into Protocol with default DNS implementations
• Add first-class PROCESSOR module to Dns_server for contextual
  protocol extensions
• Change Dns_server.listen to accept processor
• Rename Dns_server.listen_with_zonebuf and Dns_server.listen_with_zonefile
  to Dns_server.serve_with_zonebuf and Dns_server.serve_with_zonefile resp.
• Add processor_of_process, process_of_zonebuf,
  eventual_process_of_zonefile, and serve_with_processor to Dns_server
• Rename Query.query_answer to Query.answer
• Add Query.response_of_answer and Query.answer_of_response
• Move Dns_resolver.build_query to Query.create
• By default, DNS packet IDs are randomly generated with Random
• Dns_resolver now supports simultaneous resolver protocol requests
• Fix reversed multiple TXT parse bug
• Move DNSSEC implementation to <//github.com/dsheets/ocaml-dnssec>