ETCH-267 Forwarding session events to user code

Till now session events are transferred through the
stack, but were not passed to the user code.
This patch makes it possible for the user code to override
_session* events.

Change-Id: Ida6b51a215888e70a6e02df748b772c188994053

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1518241 13f79535-47bb-0310-9956-ffa450edef68
ETCH-267 Using session events in CPP Hello World example

This patch demonstrates how session event handlers can be registered
in user code.

Change-Id: I4260d875893d37141ab61fc4d4432eb3f9f3259c

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1518245 13f79535-47bb-0310-9956-ffa450edef68