Add documentation to safemath functions

Fixes #655

contracts/math/SafeMath.sol

@@ -6,6 +6,10 @@ pragma solidity ^0.4.18;
  * @dev Math operations with safety checks that throw on error
  */
 library SafeMath {
+
+  /**
+  * @dev Multiplies two numbers, throws on overflow.
+  */
   function mul(uint256 a, uint256 b) internal pure returns (uint256) {
     if (a == 0) {
       return 0;
@@ -15,6 +19,9 @@ library SafeMath {
     return c;
   }
 
+  /**
+  * @dev Integer division of two numbers, truncating the quotient.
+  */
   function div(uint256 a, uint256 b) internal pure returns (uint256) {
     // assert(b > 0); // Solidity automatically throws when dividing by 0
     uint256 c = a / b;
@@ -22,11 +29,17 @@ library SafeMath {
     return c;
   }
 
+  /**
+  * @dev Substracts two numbers, throws if subtrahend is greater than minuend.
+  */
   function sub(uint256 a, uint256 b) internal pure returns (uint256) {
     assert(b <= a);
     return a - b;
   }
 
+  /**
+  * @dev Adds two numbers, throws on overflow.
+  */
   function add(uint256 a, uint256 b) internal pure returns (uint256) {
     uint256 c = a + b;
     assert(c >= a);