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-api/src/org/briarproject/api/plugins/PluginCallback.java

@@ -49,4 +49,7 @@ public interface PluginCallback {
 	 * format string and arguments.
 	 */
 	void showMessage(String... message);
+
+	/** Schedules the plugin to be polled immediately. */
+	void pollNow();
 }