Added udp socket example

This pull requests includes a udp server/client example to get started with UDP and RIOT. It uses the posix socket API. Unfortunately it seems the ```close``` function (posix socket) is not exposed by RIOT and therefor missing.
`close` is implemented and exposed (`unistd.h`), please enable this line. 

without close the socket is not reused and the resource is wasted -> only 5 posix sockets in default config