chore(engine): upgrade default version of camunda bpm used to 7.2.0

Closes #27
fix(config): work around missing config object 'isSet' method for grails 2.3.x support

Affects #36
improve(config): resolve and clean up "provided camunda libraries" dependency issue

Closes #33
docs(readme): add some information to quickly deploy a clickable camunda BPM on grails example