New Plugin MemLog: Create in-memory logs

I was searching for a solution to generate messages which can be shown in the visualization software. My first thought was to have a simple log file which could be read and transferred to the client. But then I saw the `log` command in the `visu` plugin, which is using a in-memory log and I think this could be a good idea to solve this.

I created a new `memlog` plugin which can be used to configure different in-memory logs (see also [this](http://knx-user-forum.de/smartvisu/31449-verwendung-des-status-log-widgets.html) discussion). With this it's now possible to configure several in-memory logs for different purpose.

I was thinking about a in-memory logs for some alerts (I don't need them in a real log or in a history, I just want to show some messages in the visualization), which shows up some still open windows when entering absence status, or which shows up messages when a phone call was while absent, or heating is still turned on when windows are open, or ...

Take a look at the `README.md` file, to see what's possible with this plugin. In short, you can attach them to items to log the string message / item value and you'll be able to use them in the logics.