introduce erlang 'application' and corresponding config file

this replaces the use if '-define()' for stuff like NAT parameters
diff --git a/src/mgw_nat_app.erl b/src/mgw_nat_app.erl
new file mode 100644
index 0000000..6b43a94
--- /dev/null
+++ b/src/mgw_nat_app.erl
@@ -0,0 +1,11 @@
+-module(mgw_nat_app).
+-behavior(application).
+-export([start/2, stop/1]).
+
+start(_Type, _Args) ->
+	Sup = mgw_nat_sup:start_link(),
+	io:format("Sup ~p~n", [Sup]),
+	Sup.
+
+stop(_State) ->
+	ok.