Add constant modifier to ERC721_RECEIVED magic value

contracts/token/ERC721/ERC721BasicToken.sol

@@ -13,8 +13,9 @@ contract ERC721BasicToken is ERC721Basic {
   using SafeMath for uint256;
   using AddressUtils for address;
   
-  // Equals to bytes4(keccak256("onERC721Received(address,uint256,bytes)"))
+  // Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
-  bytes4 ERC721_RECEIVED = 0xf0b9e5ba; 
+  // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
+  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 
 
   // Mapping from token ID to owner
   mapping (uint256 => address) internal tokenOwner;

contracts/token/ERC721/ERC721Receiver.sol

@@ -8,9 +8,10 @@ pragma solidity ^0.4.18;
 contract ERC721Receiver {
   /**
    * @dev Magic value to be returned upon successful reception of an NFT
-   *  Equals to bytes4(keccak256("onERC721Received(address,uint256,bytes)"))
+   *  Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`,
+   *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
    */
-  bytes4 ERC721_RECEIVED = 0xf0b9e5ba; 
+  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 
 
   /**
    * @notice Handle the receipt of an NFT