Merge pull request #2 from mrngm/master

Add README with convert queries
Fixed multiple bugs in factuurinput
Implemented negation of amount on inkoop facturen.
Fixed bug in parse_factuur_regel
Updated policy for olympus
Fixed bug in error reporting still using old variable names
Updated policy for olympus
Merge branch 'master' of https://github.com/davidv1992/madmin
server/madmin_factuur.py: add product name to NonExistingProductException, fixes #8