Fix context errors

Author: John Haley

lib/core.js

@@ -32,15 +32,15 @@ function Promise(fn) {
   }
 
   this.value = function () {
-    if (!this.isFulfilled()) {
+    if (!self.isFulfilled()) {
       throw new Error('Cannot get a value of an unfulfilled promise.')
     }
 
     return value
   }
 
   this.reason = function () {
-    if (!this.isRejected()) {
+    if (!self.isRejected()) {
       throw new Error('Cannot get a rejection reason of a non-rejected promise.')
     }