Create lockdown test for tag cloud

The tag cloud appears to have 100% code coverage, but it turns out that
there are no assertions regarding this part of the code, it just runs
without raising any exceptions.

Run the lockdown test with `bundle exec rake test:lockdown`.

The lockdown test is an `approval` test, meaning that whatever you
previously approved is considered the correct result.

The first time you run the test there is no previously approved result,
so the test will fail.

To make it pass copy the `.lockdown/received.html` file to
`.lockdown/approved.html`. By doing this you _approve_ the current
output as being correct.

If you run `bundle exec rake test:lockdown` again, the test should pass.
iteration 2
added hack for zeus who did not find the TagCloud, made StatsController#get_stats_tags call TagCloud.compute
use TagCloud#compute results in StatsController#get_stats_tags
naming cleanup in TagCloud
extracted sql queries from TagCloud#compute
unified sql queries
use two tag clouds and remove duplication in TagCloud#compute
pass only the cloud-instances into the view
removed duplication in tags partial
extracted tags creation into own method
updated selenium-webdriver for compability with Firefox 24
refactored TagCloud
polished the view
merged in iteration10