Added customer_nr to database and invoice templates.

Used standard DATEV format(10000 – 69999)

Fixes #31

app/Http/Controllers/CustomerController.php

@@ -20,4 +20,20 @@ public function index()
             return ApiDataTransformer::snakeToCamel($customerArray);
         });
     }
-}
+
+    /**
+     * Generate a random available customer number
+     */
+    public static function generateCustomerNumber()
+    {
+        $newNumber = null;
+        while (!$newNumber) {
+            // DATEV standard
+            $randomNumber = rand(10000, 69999);
+            $customer = Customer::firstWhere('customer_nr', $randomNumber);
+            if ($customer) {
+                return $randomNumber;
+            }
+        }
+    }
+}