add memoization to .path. Closes #17

benchmarks/middleware.js

@@ -11,6 +11,7 @@ console.log('  %s middleware', n);
 while (n--) {
   app.use(function(next){
     return function *(){
+      var p = this.path;
       yield next;
     }
   });

lib/context.js

@@ -142,8 +142,8 @@ module.exports = {
    */
 
   get path() {
-    // TODO: memoize
-    return parse(this.url).pathname;
+    var c = this._pc = this._pc || {};
+    return c[this.url] || (c[this.url] = parse(this.url).pathname);
   },
 
   /**