Add helper functions to query spread and slippage in rate helper contract.

* get spread API will query rate spreads for a token. will calculate spread for all reserves listed for that token.

* get slippage: buy and sell slippage for token. for all reserves listed for that token. 

* supports default query amounts (when user sets amount 0)

* all amounts are in wei

Author: ducquangkstn

contracts/sol6/wrappers/IKyberRateHelper.sol

@@ -6,8 +6,8 @@ import "../IKyberReserve.sol";
 interface IKyberRateHelper {
     function getRatesForToken(
         IERC20 token,
-        uint256 optionalBuyAmount,
-        uint256 optionalSellAmount
+        uint256 optionalBuyAmountWei,
+        uint256 optionalSellAmountTwei
     )
         external
         view
@@ -20,8 +20,8 @@ interface IKyberRateHelper {
 
     function getPricesForToken(
         IERC20 token,
-        uint256 optionalBuyAmount,
-        uint256 optionalSellAmount
+        uint256 optionalBuyAmountWei,
+        uint256 optionalSellAmountTwei
     )
         external
         view
@@ -34,8 +34,8 @@ interface IKyberRateHelper {
 
     function getRatesForTokenWithCustomFee(
         IERC20 token,
-        uint256 optionalBuyAmount,
-        uint256 optionalSellAmount,
+        uint256 optionalBuyAmountWei,
+        uint256 optionalSellAmountTwei,
         uint256 networkFeeBps
     )
         external
@@ -46,4 +46,33 @@ interface IKyberRateHelper {
             bytes32[] memory sellReserves,
             uint256[] memory sellRates
         );
+
+    function getReservesRates(IERC20 token, uint256 optionalAmountWei)
+        external
+        view
+        returns (
+            bytes32[] memory buyReserves,
+            uint256[] memory buyRates,
+            bytes32[] memory sellReserves,
+            uint256[] memory sellRates
+        );
+
+    function getSpreadInfo(IERC20 token, uint256 optionalAmountWei)
+        external
+        view
+        returns (bytes32[] memory reserves, int256[] memory spreads);
+
+    function getSlippageRateInfo(
+        IERC20 token,
+        uint256 optionalAmountWei,
+        uint256 optionalSlippageAmountWei
+    )
+        external
+        view
+        returns (
+            bytes32[] memory buyReserves,
+            int256[] memory buySlippageRateBps,
+            bytes32[] memory sellReserves,
+            int256[] memory sellSlippageRateBps
+        );
 }