PN Invoice 0.2.0

Changed

• Breaking: the root namespace is now Pnscripts\Invoice\ (was PnScripts\Invoice\), the
  spelling every PN Scripts package uses. Behaviour is unchanged. To upgrade, replace
  PnScripts\Invoice\ with Pnscripts\Invoice\ in your use statements and type references,
  for example sed -i 's/PnScripts\\Invoice/Pnscripts\\Invoice/g' $(grep -rl 'PnScripts\\Invoice' src).
  Composer's autoloader matches the prefix case-sensitively, so the old spelling fails with
  "class not found" until it is updated.