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).
Fixed jquery import in questionset.html (the path did not match the actual file).
Added custom field to the question type dropdown list in Django Admin.
Fixed and changed the "Same as Another Question field".

This field didn't work, because there were check parse errors.
Currently, to use this field, you need to put "sameas=question.id"
in the checks field. We used the question id in order to be able
to use this functionality between different questionnaires.
Same as Another Question can be selected using question number of question id.

If you use sameas, you need to use question number within the current questionnaire.
If you use sameasid - the question ID is expected.
Remove distinct filter, this option is not supported by all database backends, and two questions with same number don't work well
Fixed errors when converting choice to unicode, if choice is an Integer.
Added 'Export to CSV' functionality to the Django Admin interface.
Added verbose_name to Question text.

Other wise it showed as "None [Languaage]" in Django Admin.
It is possible to filter questions depending on the questionnaire in the Django Admin interface.
It is possible to show all questionniare questions when adding show_all=1 GET parameter.

It is useful for debugging purposes, when you want to see all questions rendered
on one screen.

Conflicts:
	questionnaire/views.py
Made Question#extra field max_length larger (128 -> 512).

Conflicts:
	questionnaire/models.py
Bugfix - iftag="" did not work for a question that is in a questionset, in which other questions should be visible.

In that case, even though the question did not fulfill the iftag requirement it was shown.

Conflicts:
	questionnaire/views.py
Substitute_answer throwed an exception for empty answer values.
Small fixes and updates in the README.md file.