feat(engine): enable async start events

- a start event may be declared async such that on execution, a new
process instance is created, but no activity executed yet

related to CAM-424
test(engine): test execution listener invocation on async start

related to CAM-424
docs(engine): improve JavaDocs of TaskService and Task
fix(webapps/login): improve error messages on no app access

- The inability to access an app (due to missing credentials) gets
properly
  signaled via HTTP 403 (FORBIDDEN)
- The login failed method reflects the matter, too
- The internals of the security filter are changed to report the fact,
  whether a user is authorized or not to be able to properly
  handle an authentication error

Related to CAM-1059
feat(engine): add <plugins ...> to configuration

- add possibility to configure process engine plugins in bpm-platform.xml
  and processes.xml

related to #CAM-978
feat(filter): Show filter in process instance view

- the sitebar contains now a filter dialog, which shows the current number
  of selected activity instances and selected activities

related to CAM-1094
Merge branch 'taskservice-javadoc' of git://github.com/falko/camunda-bpm-platform into merge
refactor(authentication): replace cookie-auth with X-Authenticated header

- instead of relying on a client-side cookie we now rely on a number of
  X-Authenticated headers that contain the currently authenticated user.

- whenever the header has changed (is added or removed) in between
  successive requests, a change in the server-side login state gets
  apparent. The client application will reflect that change in the UI.

- Serverside authentication has a mechanism now to distinguish
  logged-in but forbidden (403) from unauthorized (401) requests.
  This is achieved via the
  org.camunda.bpm.webapp.impl.security.filter.Authorization class
  that contains request relevant authorization data.

- A few test cases test the basic login behavior. More stuff must happen
  here though.

Related to CAM-1059
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
fix(messages): improve task complete / process start error message

Related to CAM-1059
fix(admin/messages): add failure notices for user / group add / create

Related to CAM-1059
fix(engine): fix execution listener invocation on async start event

related to CAM-424
improve(engine): support null configuration in async job handler

- supports migrated jobs that do not have a configuration value set

related to CAM-424
Merge branch 'CAM-424'

Conflicts:
	engine/src/main/java/org/camunda/bpm/engine/impl/persistence/entity/ExecutionEntity.java
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
feat(ldap): add ldap library and config to distros

- add ldap library to all distributions
- make sure jboss subsystem instantiates process engine plugins

related to #CAM-978
Merge branch 'master' of github.com:camunda/camunda-bpm-platform

Conflicts:
	webapps/camunda-webapp/webapp/src/main/java/org/camunda/bpm/webapp/impl/security/auth/AuthenticationFilter.java
feat(ldap): add button in webapp

- add "Setup LDAP" button in Webapp

related to #CAM-819
refactor(cockpit/instance-tree): rework internals and look + feel

Closes CAM-1094.
Merge branch 'master' of github.com:camunda/camunda-bpm-platform