Improve wsus accuracy

[meeehow]

This PR makes the following changes:

1. Add Description() function to the source interface, which allows to add additional description of the source.
2. Modify PostgreSQL schema to add sourceDescription field to the sources table
3. Check if "export.csv" file is present in the root of the WSUS repo, if yes then download this file and use it to map the WSUS update file (sha1) with the actual upadate name (which contains KB number)
4. Adjusts tests