MachineEntity extends EmptySoftwareProcess

Otherwise it's not constructable
Manually add existing machines to server pools
Handle null locations in LocalLocationManager and correct JcloudsByonLocationResolver Javadoc
ServerPools refuse to remove manually-added machines until they are stopped
Synchonise accesses of attributes in ServerPoolImpl.stop
Convert Groovy to Java
Consume process stderr in addition to stdout so it doesn't block.

If there is any error output throw an exception.
Use alternative approach to find unused PID.

ps ax appears to fail on builds.apache.org, use alternative
approach of finding an unused PID which even is more effective.
This closes #89
This closes #95
This closes #96