KAUFDA-6129 Swiss - Datamining Coupon KPI's

Fix for #3 and #4. I've included a regression test for #3. You can verify that this test reproduces #3 by temporarily changing line 58 of `NonBlockingStatsDClient` from:

````java
NumberFormat numberFormatter = NumberFormat.getInstance(Locale.US);
````

to 

````java
NumberFormat numberFormatter = NumberFormat.getInstance();
````

which is consistent with the previous version. After making this change the regression test will fail. Could you please make a new release ASAP after this pull request has been merged, because currently this library is not usable in any locale that uses something other than a period as the decimal separator (Germany, France, etc.).