Add UDP tests
These tests explore the interaction between scalanet and an external library under UDP. UDP protocol allows to leave the source port of a datagram to be dynamically allocated. In scalanet, the source port is fixed in order to support a notion of addressing/multiplexing. Hence, the recipient of a Channel assumes that it can reply to the sender through the received channel. This difference leads to inconsistent behaviors with respect to UDP implementations. The tests show that a server written using scalanet would fail to reply to a client using the intuitive interface. However, it would be able to reply if it ignores the channel received.