Separate Agent MBean from Output

The JaCoCo agent should register an MBean separately to the output mode. This allows to use the MBean in combination with an output mode (e.g. trigger dump to file via MBean). The MBean will then have the same API as the runtime API defined in #61: 

  * Get agent version
  * Set/get session id
  * reset execution data
  * Dump (and optionally reset) execution data via configured output 
  * Dump (and optionally reset) execution data to byte[]

The MBean is registered by default. This behavior can be modified with the new boolean agent option mbean (e.g. mbean=false).