Update StandardToken.sol

Added increaseApproval and decreaseApproval to increase / decrease the approval in 1 transaction.

contracts/token/StandardToken.sol

@@ -64,4 +64,31 @@ contract StandardToken is ERC20, BasicToken {
     return allowed[_owner][_spender];
   }
   
+    /*
+   * approve should be called when allowed[_spender] == 0. To increment
+   * allowed value is better to use this function to avoid 2 calls (and wait until 
+   * the first transaction is mined)
+   * From MonolithDAO Token.sol
+   */
+  function increaseApproval (address _spender, uint _addedValue) 
+    onlyPayloadSize(2 * 32)
+    returns (bool success) {
+        allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
+        Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
+        return true;
+    }
+
+  function decreaseApproval (address _spender, uint _subtractedValue) 
+    onlyPayloadSize(2 * 32)
+    returns (bool success) {
+        uint oldValue = allowed[msg.sender][_spender];
+        if (_subtractedValue > oldValue) {
+            allowed[msg.sender][_spender] = 0;
+        } else {
+            allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
+        }
+        Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
+      return true;
+  }
+
 }