Add uppercase filter.

Author: addyosmani

filter-uppercase.js

@@ -0,0 +1,8 @@
+/**
+ * Transform an input string to uppercase
+ * @param  {string} input
+ * @return {string} uppercase string
+ */
+PolymerExpressions.prototype.uppercase = function(input){
+  return input.toUpperCase();
+};