(PDB-671) Enable JMX support for Jetty 9

This patch enables JMX support for exposing Jetty 9 metrics.

I've introduced a new configuration item `jmx-enable` which is a string based
boolean configuration item. It defaults to `true` which enables JMX support
by default.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #45 from kbarber/ticket/master/pdb-671-jetty-jmx-support

(PDB-671) Enable JMX support for Jetty 9