Persist worth data

[LittleBigBug]

My idea to go forward with this plugin is more of a stats plugin. I think the things that should be added are: automatic tracking of worth periodically and store it permanently for that period. (Currently, worth is only calculated when a player checks an items' worth, and is cached until the item sells again, after that the old calculated worth is lost). So, instead check and calculate this data periodically and save that in another table.

Eventually it would be cool to create some graph report of this data, however I think the best way to go about that would be to create a separate web interface that interacts with the AcoWorth database directly so another webserver isn't ran, requiring an extra port like DynMap.