Added new columns in review:
* content_license
* source - review source (used in importing)
* source_url - URL of review source
Added new arguments for create() and update() functions in Review to simplify importing.
Added a note about review origin and license on review page.
Added a note about get_or_create() method usage in User model.
Adjusted code style a bit.
Fixed argument name in review updater.
Added DEFAULT_LICENSE constant.