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.

MotD Feature:

There are two types of MotD: 
AdminMessages
SessionMessages

Every Message has a title, text, start- and enddate.

You can declare an admin account in the arsnova.properties (security.adminaccount = <adminusername>) and with this account you can create Messages for different audiences (different types are: all (shown on startpage), loggedIn, students, tutors) which will be shown in a messageBox.

SessionMessages can be created for every session by the sessioncreator and is displayed, when you enter the session.

When a user that is not logged in via Guest, he pushes a list to the backend with all the messagekeys he read and acknowledged, so that he isn't shown the same messages over and over again. In addition to that, the keys are also stored in the localStorage (e.g. for guest accounts).

See merge request !15