change Math to a library instead of a contract

contracts/math/Math.sol

@@ -5,7 +5,7 @@ pragma solidity ^0.4.11;
  * @dev Assorted math operations
  */
 
-contract Math {
+library Math {
   function max64(uint64 a, uint64 b) internal constant returns (uint64) {
     return a >= b ? a : b;
   }