SQLite plugin: new delta column containing difference between previous value

This changes the SQLite version to `3` and adds a new `delta` column to the `history` table. This column will contain the delta (differences between the current and the previous value).

It will also add a new series function called `delta` which returns these values. This could be useful to generate a chart in the visualization showing the delta instead the average value of an item (e.g. when logging the outdoor temperature you can easily create a chart showing the increment/decrement of the temperature for a time range).

Don't know if it's really useful, but I can't find any other way to retrieve a delta for an item (instead of creating an own item for the delta calculation, which might be a little bit more complex).

Currently not tested very much, but the current logging functionality works without problem. The packing was not executed so far, but should work too.

What do you think?