Using SmartLifecycle to register MBeans

Prior to this commit, MBeans were registered in a post construct
call of MBeanExporter. This commit moves that logic after the
initialization phase using the SmartLifecycle callback.

Issue: SPR-8045
polishing

Reorganized class structure to match our code style (setter for
properties at the top of the class, public method before private
implementation).

Removed DisposableBean as it the lifecycle is already taking care
of removing MBeans on stop.

Cleaned test suite

Issue: SPR-8045
Merge pull request #551 from mdeinum/SPR-8045

* SPR-8045:
  polishing
  Using SmartLifecycle to register MBeans