User Balance Calculator



Changes based on Rohans comments in #311...
1) Remove uncessary changes. Although there are unnecessary commits on this PR the changed files are the right ones. Is this ok? 
2) Tweaks and clean up.
3) Removed passing objects from method call in calculator. Obviously it makes sense to only pass in method or constructor. Given the calculator makes no sense without both distributor and customer objects best use them in initialisation.

Thanks!