Minor bug fixes, Ranking database and keyword analysis

Yo Wim,

Here is a pull request containing the following updates:

 - Minor bug fixes regarding Python version 3 calling convention.
 - Rankings can now be deleted from the database and some clarification regarding its use.
 - Keywords analysis dump added to make some further analysis of the CVEs (e.g. http://www.foo.be/cve/) including the updated requested from Twitter discussions (https://twitter.com/adulau/status/318681856465829888).

Enjoy.