Poll plugins when connectivity changes. Bug #66.

This should enable us to connect to contacts faster at startup and
whenever a new means of connecting becomes available.

briar-tests/src/org/briarproject/plugins/tcp/LanTcpPluginTest.java

@@ -133,6 +133,8 @@ public class LanTcpPluginTest extends BriarTestCase {
 
 		public void showMessage(String... message) {}
 
+		public void pollNow() {}
+
 		public void incomingConnectionCreated(DuplexTransportConnection d) {
 			connectionsLatch.countDown();
 		}