Additional checks for validity and in validator

* Properly check nbf and exp claims in token to determine simple
  validity.
* Properly check nbf and exp claims in validator and throw exceptions
  if expired/not yet valid.

src/JWTToken.php

@@ -70,6 +70,14 @@ class JWTToken
                 $this->valid = false;
             }
         }
+
+        if ($this->header->has('nbf')) {
+            $nbf = intval($this->header->get('nbf'));
+            if ($nbf >= time()) {
+                // Invalid if before
+                $this->valid = false;
+            }
+        }
     }
 
     /**