Swimlanes modeler support

Initially it was  branch to add swimlanes support. Model had a support for swimlanes, but it was incomplete - for example sequence flows was not stored in case we used pools & lanes, was not supported a case then bpmn file containts pools and porcesses inside pool, but these processes does not have any lane.
Also we added some more "related" elements support: MessageFlow, SendTask.

Additionally we added support for new elements into stencilset.json - but not sure we should push it into main repository - since in that case generated processes may be not executed in activiti engine (since it has limited support for pools).

New version with these changes is already deployed into our project: https://processo-modeler.appspot.com