added increase and decrease approval.

Author: vinay035

contracts/SRTToken.sol

@@ -81,7 +81,7 @@ contract StandardToken is ERC20 {
         //  allowance to zero by calling `approve(_spender, 0)` if it is not
         //  already 0 to mitigate the race condition described here:
         //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
-        require((_value == 0) || (allowed[msg.sender][_spender] == 0));
+        require(allowed[msg.sender][_spender] == 0);
         allowed[msg.sender][_spender] = _value;
         Approval(msg.sender, _spender, _value);
         return true;
@@ -96,6 +96,29 @@ contract StandardToken is ERC20 {
     function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
       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) public returns (bool) {
+        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) public returns (bool) {
+        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;
+    }
 }
 
 contract Ownable {