added luhn algorithm

Author: AlexDvorak

Checksums/luhn.js

@@ -0,0 +1,16 @@
+function luhn(card_number) {
+    var sum_of_odds  = 0;
+    var sum_of_evens = 0;
+    for (var i = 15; i > 0; i-=2) {
+        sum_of_odds += parseInt(card_number[i]);
+    }
+    for (var i = 14; i > -1; i-=2) {
+        var current_number = parseInt(card_number[i]);
+        if (parseInt(card_number[i])*2 > 9) {
+            sum_of_evens += (i*2)-9;
+        } else {
+            sum_of_evens += i*2;
+        }
+    }
+    return 0 === ((sum_of_odds + sum_of_evens) % 10);
+}