Added udp posix 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.
in case the application transfers `[abc], bytes=3`, the string are not terminated by 0.
make sure the string is terminated on printf, or use:
`printf("%.*s", stringLength, pointerToString);`