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.
this `+1` is a workaround, right:)

there is a problem in the stack, which you worked around in native....native zeros all buffers....I'm going to investigate this

DO NOT MERGE NOW <- does not work on msba2, in case string length is odd...