add a MySQL table for keeping ratings
implement basic rating
include item rating in description
allow filtering for item rating

Support exact, min and max rating filters where
the last two can be combined, but the first has
precedence. Some fixes to existing code were re-
quired due to SQL changes.
implement item rating configuration
change URL and file name
item ratings: allow individual retrieval

Make it possible to retrieve a list indicating how
the individual users rated the item.
Merge branch 'ratings' - implement item rating - closes #10