Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Additionally, removed unused empty constructor

Additionally, remove unused loggers.

Changed `security.facebook.enabled-roles` to
`security.facebook.allowed-roles`.

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

This simplifies analysis of monitoring data.

Configuration for Socket.IO has been refactored to support proxies and
replace ambiguous property names:
* Added API property `socketioPath` to `/configuration/`
* Added property `socketio.proxy-path`
* Removed property `security.ssl`
* Renamed properties:
  * `socketio.ip` -> `socketio.bind-address`
  * `security.keystore` -> `socketio.ssl.jks-file`
  * `security.storepass` -> `socketio.ssl.jks-password`

The default port to bind to has been changed from 10443 to 8090 since
SSL is not enabled by default. There is enough room (8081-8089) for
additional severs (e.g. `grunt run`) in development environments.

The security.*.enabled properties are now loaded as booleans.

Fixes failing start up because of no longer existent
`security.guest.lecturer.enabled` setting.

fix bug where abstention questions are not exported

implement: via two booleans you can now decide whether AnswerStatistics and FeedbackQuestions shall be exported, too

add constructors to transport classes to parse "main"class