Add log-level property to enable different log levels for the examples.

Motivation:
When running the examples using the provided run-examples.sh script the
log level is 'info' level. It can be handy to be able to configure a
different level, for example 'debug', while learning and trying out the
the examples.

Modifications:
Added a dependency to logback-classic to the examples pom.xml, and also
added a logback configuration file. The log level can be configured by
setting the 'log-level' system property, and if that property is not set
the default will be 'info' level.
The run-examples.sh was updated to show an example of using the system
property to set the log level to 'debug'

Result:
It is now possible to turn on debug logging by settnig a system property
on the command line.
why do we need logback at all ?