fix for "sales.paid" scope
added time span limiting for listed sales
error handling still has to be done (how to do notices?)
dates have to be of format dd.mm.yyyy hh:mm
if not, default dates will be applied
added error handling to sale date limits
Merge branch 'master' of https://github.com/bastli/bastli-pos
changed ui in sales; still to be changed properly!
added total value of all selected sales
changed default for field-clearig behavior when adding a new article (issue #11)
fixed bug where list of found articles would not be resetted uppon canceling the addition of a new article
Removed cached total from sale, not really needed and bloats code. Added statistics page, quite db intensive computations, that is why we don't want that in our general sales overview. We could add some nice charts to the statistics page, if someone has some spare time? Removed total line from sales overview as it leads to confusion when using multiple pages (only 30 sales are shown on one page)