Django-review

- fixed empty categories raising an exception for not being in the averages
dictionary in the render_category_averages template tag.

- fixed iteration over dictionary

- fixed query for ratings, that did not filter for review
- fixed bug in render_category_averages template tag

- added question field to RatingCategory to allow showing more explainatory
information. The name field remains as label for the form's category fields
as short version of the question.
- added get_review_count template tag
- added render_category_averages template tag
- fixed average calculation for 0 values

- added user_has_reviewed template tag
- allow callables for REVIEW_UPDATE_SUCCESS_URL
- made ordering of custom choices make more sense
- added setting to override the default form select widget

- re-implemented average calculation
- added total_review_average template tag
- added RatingCategoryChoice model and added counts_for_average to
RatingCategory.