Now cash is divided by upgrades price

Instead of subtracting the price of upgrades from the cash, it is
divided by it. This makes cash go down considerably when an upgrade is
bought.

js/exponential.js

@@ -53,7 +53,7 @@ angular.module('incremental',[])
 
         $scope.buyMultiplierUpgrade = function(number) {
             if ($scope.player.currency.comparedTo($scope.player.multiplierUpgradePrice[number]) >= 0) {
-                $scope.player.currency = $scope.player.currency.minus($scope.player.multiplierUpgradePrice[number]);
+                $scope.player.currency = $scope.player.currency.div($scope.player.multiplierUpgradePrice[number]);
                 $scope.player.multiplier = $scope.player.multiplier.plus($scope.multiplierUpgradePower[number]);
                 $scope.player.multiplierUpgradeLevel[number]++;
 				// The cost function is of the form 2^1.x^(upgradeLevel), where 1.x depends on the upgrade tier