upload of quizes through backend

[tillmo]

There should be a possibility to upload a structured quiz with questions directly into the backend database, by issuing a command on the server, or by calling the RESTful API. This can also be used to populate the database with initial data, see #20. The structure should be one text for a quiz with all its questions, answers, justifications and explanations, such that there is no need to work with IDs for referencing. The text should have the following form:

quiz: name of
d: description of quiz
q: question 1
a: answer 1*
j: justification 1
e: explanation 1
j: justification 2*
a: answer 2
...
q: question 1
...

Correct answers and justifications are marked with a *.

[tillmo]

changed from json to structured text, because this can be written more easily. Processing it should be easy, too.

[tillmo]

fixed by #57