Fixing issue with configuration based stubbing of extras content.
Make use of `os.tmpdir()` to load ".popular.json".

This way it should work on every OS that has the environment variable set.
Merge pull request #390 from XhmikosR/extras-cross-platform

Make use of `os.tmpdir()` to load ".popular.json".
extras_popular.jade: Add jQuery Tablesorter plugin.

This allows us to sort the table.

TODO: Get rid of the empty `th`.