chore(rest): extend process instantiation example
fix(bpmn reference): correct link to event-based gateway
feat(message event): document engine's correlation mechanism

closes CAM-666
chore(engine): update rest api to fit refactored group info method

closes CAM-661
feat(rest): add docs for message correlation method

- closes CAM-697
docs(WAS): platform installation instructions

related to CAM-441
added structure for best practices as starting point for discusssion
added some more example content
removed accidentially on master committed changes for best practices section
fix(docs): remove obsolete steps
improve(rest): rename 'start process' to 'start process instance'
chore(rest): comment out POST /message docs for release
upd(userguide): new Ejb PA example
chore(userguide): rename PA
chore(rest): readd docs on POST /message
fix(tomcat): Add missing listiner in docs

- Add the doc to add the tomcat bootstrap listener to bootstrap the
  camunda BPM platform on startup

closes CAM-681
feat(WAS): more and better embedded images. add RAR properties hint.

related to CAM-441