Ensure no duplicate keys (#24)

When a key is held down such that the OS sends repeats, "keydown" will be called before "keyup", so we need to ensure that the key is not present in the array before inserting it.

Author: MonoidMusician

src/FRP/Event/Keyboard.js

@@ -2,7 +2,10 @@
 
 var currentKeys = [];
 addEventListener("keydown", function(e) {
-  currentKeys.push(e.keyCode);
+  var index = currentKeys.indexOf(e.keyCode);
+  if (index < 0) {
+    currentKeys.push(e.keyCode);
+  }
 });
 addEventListener("keyup", function(e) {
   var index = currentKeys.indexOf(e.keyCode);