fix(cockpit tests): exclude commons-io dep
feat(cockpit plugins): support webapp provided assets

Assets of internal plugins may be served directly by the cockpit
application.

- Specifying 'webapp://some/path' as a asset directory will serve
  client side plugin assets through the applications web resources.

- Plugin registry has been refactored to be an instance rather than
  a static field holder. That was neccessary due to a bug in the
arquillian
  test runner.
Merge branch 'master' of github.com:camunda/camunda-bpm-platform