Make SafeMath a library

contracts/SafeMath.sol

@@ -4,7 +4,7 @@ pragma solidity ^0.4.8;
 /**
  * Math operations with safety checks
  */
-contract SafeMath {
+library SafeMath {
   function safeMul(uint a, uint b) internal returns (uint) {
     uint c = a * b;
     assert(a == 0 || c / a == b);