Changes

Add GSMTAP logging handler

This implements a logging handler [1] for Erlang's built-in logger,
that sends logging messages via GSMTAP (udp/4729) and allows to
capture logging together with the S1AP PDUs.

The new handler is added to the example configuration, sending
all logging messages (DEBUG and above) to 127.0.0.1 by default.

We may want to split this up into a separate project (library).

[1] https://www.erlang.org/doc/apps/kernel/logger_handler.html

Change-Id: Iea884e2ca146b852c64bd9f135d8c71f4c925f09