serve 404 when not possible key

lib/document_handler.js

@@ -78,6 +78,11 @@ DocumentHandler.prototype.chooseKey = function(callback) {
   }); 
 };
 
+// Return a boolean indicating whether or not something can be a key
+DocumentHandler.potentialKey = function(key) {
+  return key.match(/^[a-zA-Z0-9]+$/);
+};
+
 // Generate a random key
 DocumentHandler.prototype.randomKey = function() {
   var text = '';