chore(signavio-it): update models

- models now contain extension elements
chore(project): add parent pom

This commit adds a parent pom that allows it to build engine projects
independently from the web applications and a configured nexus proxy
(just building the parent + the engine will do it).

Related to CAM-1716
chore(project): cleanup modules published to maven central

This commit removes all projects that are not required for building
process applications from the sonatype-oss-release profile in the root
pom.xml.

Related to CAM-1716
chore(project/developers): use community instead of individuals

This commit removes individual developers from the developers section of
our main projects in favor of a link to the camunda BPM community
(camunda.org).

Related to CAM-1716
fix(engine/persistence): OptimisticLocking only for HasRevision

make sure optimistic locking check is only performed for entities
providing a revision.

related to #CAM-1719
chore(project): add camunda nexus for projects that require it

This commit adds the camunda public nexus group as a repository to

* jbossas7 assembly
* tomcat assembly
* cycle

because it serves artifacts that are only available from there.

Related to CAM-1716
chore(project): add bill of material

This commit adds the bill of material for public camunda bpm artifacts
and core dependencies (mybatis, uuid generator, joda-time).

It can be included a project to use compatible versions of libraries for
a specific camunda BPM release.

Related to CAM-1716
chore(project): merge changes
chore(MI/LoopVariable): check whether loop variable is null

- if the loop variable is null a ProcessEngineException will be thrown.

related to CAM-1721
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
chore(engine): harmonize receive task event subscription

the subscription of a receive task can be correlated like ordinary events

Closes #CAM-1612
Merge branch 'master' into CAM-1712-grunt