Tags are added/removed according to the selected CHOICEs.

It is now possible to set tags that need to be enabled if
a user selects some choice from the choice questions.
The tags from the other, non-selected choices are deleted
(in case of somebody changing the previously saved values).
Added a NUMBER question type.

(Uses the same code as the already implemented RANGE question type).
It is possible to define some code to be executed while saving an answer to the question.

E.g. on saving an answer to some question we may want to modify the current tag set.
We can do that by adding some code to the database which can do that.