Added toString() method to TransportId.

briar-api/src/org/briarproject/api/TransportId.java

@@ -34,4 +34,9 @@ public class TransportId {
 	public int hashCode() {
 		return id.hashCode();
 	}
+
+	@Override
+	public String toString() {
+		return id;
+	}
 }