Added board statistics.
Statistics can be accessed with a link placed in the footer in the board view. Statistics are automatically updated, data is retrieved by an asynchronous request.

$ Fixed layout issue (add tag input).
Only one running instance of update command is allowed.
The solution uses tendo package.
Merge branch 'dev-stats' into dev
Fixed several errors related to saving threads.

$ Empty threads should not be created anymore.
$ Multiple threads should not be created anymore (this had more to do with running multiple instances of the update command at the same time but this is now ensured on a database level).
$ Stats view does not display NaN in the several fields if the board is empty.
Added global statistics.

+ Global statistics.
+ Universal template for global and per board statistics.
* Functions getting data for statistics moved to a separate file.
* Changed data url.
Added csrf protection.

+ Added ensure_csrf_cookie decorator to the ThreadView.
* Request method for save thread, add tag and remove tag hanged to POST.
* Changed several urls.
+ jquery.cookie plugin.
Fixed layout issue.