Refac export

This reworks the session import/export to generate the json via backend for performance.

See merge request !16

Conflicts:
	src/main/java/de/thm/arsnova/controller/LecturerQuestionController.java

fix bug where abstention questions are not exported

fix bug where not all attributes from grid questions are exported

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

add empty test methods

add constructors to transport classes to parse "main"class

Upgrading Netty-socketio makes ARSnova compatible to newer versions of
the official socket.io client.

This fixes a potential NPE and should slightly improve performance.

See also:
003b064b

Also added documentation for the property.