Fixed tj#180 (memory leak)

Fixed tj#180 by switching to a procedural implementation instead of
building up an entire promise chain of all the values yielded by the
generator.

index.js

@@ -41,7 +41,13 @@ co.wrap = function (fn) {
 function co(gen) {
   var ctx = this;
   if (typeof gen === 'function') gen = gen.call(this);
-  return Promise.resolve(onFulfilled());
+  var resolve, reject;
+  var result = new Promise(function(myresolve, myreject) {
+    resolve = myresolve;
+    reject = myreject;
+  });
+  onFulfilled();
+  return result;
 
   /**
    * @param {Mixed} res
@@ -54,9 +60,9 @@ function co(gen) {
     try {
       ret = gen.next(res);
     } catch (e) {
-      return Promise.reject(e);
+      return reject(e);
     }
-    return next(ret);
+    next(ret);
   }
 
   /**
@@ -70,9 +76,9 @@ function co(gen) {
     try {
       ret = gen.throw(err);
     } catch (e) {
-      return Promise.reject(e);
+      return reject(e);
     }
-    return next(ret);
+    next(ret);
   }
 
   /**
@@ -85,7 +91,7 @@ function co(gen) {
    */
 
   function next(ret) {
-    if (ret.done) return Promise.resolve(ret.value);
+    if (ret.done) return resolve(ret.value);
     var value = toPromise.call(ctx, ret.value);
     if (value && isPromise(value)) return value.then(onFulfilled, onRejected);
     return onRejected(new TypeError('You may only yield a function, promise, generator, array, or object, '