Add support for UDP (closes #33)

API Changes
-----------

The port notation is extended to support "/udp" or "/tcp" at the *end*
of the specifier string (and defaults to tcp if "/tcp" or "/udp" are
missing)

`docker ps` now shows UDP ports as "frontend->backend/udp". Nothing
changes for TCP ports.

`docker inspect` now displays two sub-dictionaries: "Tcp" and "Udp",
under "PortMapping" in "NetworkSettings".

Theses changes stand true for the values returned by the HTTP API too.

This changeset will definitely break tools built upon the API (or upon
`docker inspect`). A less intrusive way to add UDP ports in `docker
inspect` would be to simply add "/udp" for UDP ports but it will still
break existing applications which tries to convert the whole field to an
integer. I believe that having two TCP/UDP sub-dictionaries is better
because it makes the whole thing more clear and more easy to parse right
away (i.e: you don't have to check the format of the string, split it
and convert the right part to an integer)

Code Changes
------------

Significant changes in network.go:

- A second PortAllocator is instantiated for the UDP range;
- PortMapper maintains separate mapping for TCP and UDP;
- The extPorts array in NetworkInterface is now an array of Nat objects
  (so we can know on which protocol a given port was mapped when
  NetworkInterface.Release() is called);
- TCP proxying on localhost has been moved away in network_proxy.go.

localhost proxy code rewrite in network_proxy.go:

We have to proxy the traffic between localhost:frontend-port and
container:backend-port because Netfilter doesn't work properly on the
loopback interface and DNAT iptable rules aren't applied there.

- Goroutines in the TCP proxying code are now explicitly stopped when
  the proxy is stopped;
- UDP connection tracking using a map (more infos in [1]);
- Support for IPv6 (to be more accurate, the code is transparent to the
  Go net package, so you can use, tcp/tcp4/tcp6/udp/udp4/udp6);
- Single Proxy interface for both UDP and TCP proxying;
- Full test suite.

[1] https://github.com/dotcloud/docker/issues/33#issuecomment-20010400