plugin improvements

includes:
  * added default values for plugin related properties (credits go to
    @dacrome)
  * loading plugin after bean initialization
  * fail deployment when `plugin.enabled` is `true` but `plugin.jar.path`
    or `plugin.classname` are not set
  * added `NoopPlugin` as default plugin
  * added default properties to
    `src/main/deploy/addon-self-administration.properties`

see individual commits for further details.

this makes #53 some kind of obsolete