Signed SafeMath (#1559)

* signed safe math

* fix lint errors

* refactor overflow checks and add descriptions

* remove incorrect description

* add test for reversed arguments in multiplication test

* fix power operator

* improve multiplication test descriptions

* Update SafeMath.test.js

* add feature to changelog

contracts/mocks/SafeMathMock.sol

@@ -1,25 +1,43 @@
 pragma solidity ^0.4.24;
 
+
 import "../math/SafeMath.sol";
 
+
 contract SafeMathMock {
-    function mul(uint256 a, uint256 b) public pure returns (uint256) {
+    function mulUints(uint256 a, uint256 b) public pure returns (uint256) {
         return SafeMath.mul(a, b);
     }
 
-    function div(uint256 a, uint256 b) public pure returns (uint256) {
+    function mulInts(int256 a, int256 b) public pure returns (int256) {
+        return SafeMath.mul(a, b);
+    }
+
+    function divUints(uint256 a, uint256 b) public pure returns (uint256) {
         return SafeMath.div(a, b);
     }
 
-    function sub(uint256 a, uint256 b) public pure returns (uint256) {
+    function divInts(int256 a, int256 b) public pure returns (int256) {
+        return SafeMath.div(a, b);
+    }
+
+    function subUints(uint256 a, uint256 b) public pure returns (uint256) {
         return SafeMath.sub(a, b);
     }
 
-    function add(uint256 a, uint256 b) public pure returns (uint256) {
+    function subInts(int256 a, int256 b) public pure returns (int256) {
+        return SafeMath.sub(a, b);
+    }
+
+    function addUints(uint256 a, uint256 b) public pure returns (uint256) {
         return SafeMath.add(a, b);
     }
 
-    function mod(uint256 a, uint256 b) public pure returns (uint256) {
+    function addInts(int256 a, int256 b) public pure returns (int256) {
+        return SafeMath.add(a, b);
+    }
+
+    function modUints(uint256 a, uint256 b) public pure returns (uint256) {
         return SafeMath.mod(a, b);
     }
 }