New Plugin DataLog: Provide easy logging mechanism for items

Usually I use a SQLite database to store all the values and use them to show up some charts in my visualization. The problem with this is, that the data is packed at a certain interval. This will aggregate the information to minimize the database size (e.g. houly data is aggregated to daily, daily to weekly, ...).

This plugin will provide functionality to log the item status into log files to have the item status history available - months or years ago.

This could be useful if you want to still have the data available month ago to do a analyzing task or to import them into other database system or data container (e.g. Excel).

I wrote this plugin, since I still want to keep the detailed item data (e.g. from my solar plant) to still be able to do some further analyzing. I feel better when I know the data is somewhere in the logs, instead of thinking about how to use the aggregated values from the SQLite database - which is IMHO only a performance optimized data container to provide information on a fast way (e.g. for the visualization).

Btw. I also found a [conversation](http://knx-user-forum.de/320568-post10.html) about such a feature to be able to keep some values in the SQLite database. Since I don't want to blow up the SQLite database that much with detailed history data, I think such a plugin could be better.