Fix for jmxutils 1.14

Martin now insists on exporting the MBeanExporter in the MBeanModule.
Intercept this with a module override. Better fix would be a
lifecycled thingie that gets the exporter injected and calls unexport
instead of extending the MBeanExporter.
Merge pull request #6 from hgschmie/jmx-fix

Fix for jmxutils 1.14