Add Modulo operation for getting the quotient (#915)

* Add Modulo operation for getting the quotient

* Improved modulo tests and implementation.

* Removed assertion.

contracts/math/SafeMath.sol

@@ -54,4 +54,13 @@ library SafeMath {
 
     return c;
   }
+
+  /**
+  * @dev Divides two numbers and returns the remainder (unsigned integer modulo),
+  * reverts when dividing by zero.
+  */
+  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
+    require(b != 0);
+    return a % b;
+  }
 }