UDP socket support for Windows Runtime.

This implementation comes with the same caveats as the TCP implementation,
with the exception that explicit binding is supported.

The following socket options are supported:
- IP_TTL
- IPV6_UNICAST_HOPS
- SO_RCVBUF (only Windows 8.1+)
- IP_DONTFRAGMENT (only Windows 8.1+)

It has been tested (as Boost.Asio based on Boost 1.55) with UDPv4
on Visual C++ 2012 (Windows 8.0) and Visual C++ 2013 (Windows 8.1).