Remove underscores from event parameters. (#1258)

* Remove underscores from event parameters.

Fixes #1175

* Add comment about ERC

contracts/mocks/ERC721ReceiverMock.sol

@@ -8,11 +8,11 @@ contract ERC721ReceiverMock is IERC721Receiver {
   bool internal reverts_;
 
   event Received(
-    address _operator,
-    address _from,
-    uint256 _tokenId,
-    bytes _data,
-    uint256 _gas
+    address operator,
+    address from,
+    uint256 tokenId,
+    bytes data,
+    uint256 gas
   );
 
   constructor(bytes4 _retval, bool _reverts) public {

contracts/token/ERC721/IERC721Basic.sol

@@ -40,19 +40,19 @@ contract IERC721Basic is IERC165 {
    */
 
   event Transfer(
-    address indexed _from,
-    address indexed _to,
-    uint256 indexed _tokenId
+    address indexed from,
+    address indexed to,
+    uint256 indexed tokenId
   );
   event Approval(
-    address indexed _owner,
-    address indexed _approved,
-    uint256 indexed _tokenId
+    address indexed owner,
+    address indexed approved,
+    uint256 indexed tokenId
   );
   event ApprovalForAll(
-    address indexed _owner,
-    address indexed _operator,
-    bool _approved
+    address indexed owner,
+    address indexed operator,
+    bool approved
   );
 
   function balanceOf(address _owner) public view returns (uint256 _balance);