plugins/ip-faces: Implementing UdpFace and fixing bugs with TcpFace

Now UdpFace is fully working. Due to nature of UDP transmission, all
demultiplexing is happening inside IpFaceStack (with TCP, NS-3 is taking
care of that).

Bugfix: Map between IP addresses and Tcp/Udp faces was incorrectly
embedded as a static variable inside Udp/TcpFace implementation, causing
unexpected behavior.

Refs #1006 (http://redmine.named-data.net/)
diff --git a/examples/wscript b/examples/wscript
index c5c35eb..08f6f09 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -98,3 +98,6 @@
     if 'ip-faces' in bld.env['NDN_plugins']:
         obj = bld.create_ns3_program('ndn-simple-tcp', all_modules)
         obj.source = 'ndn-simple-tcp.cc'
+
+        obj = bld.create_ns3_program('ndn-simple-udp', all_modules)
+        obj.source = 'ndn-simple-udp.cc'