Log TransactionExecution event when transaction is successfully executed

contracts/exchange/contracts/src/MixinTransactions.sol

@@ -135,6 +135,8 @@ contract MixinTransactions is
             currentContextAddress = address(0);
         }
 
+        emit TransactionExecution(transactionHash);
+
         return returnData;
     }
 

contracts/exchange/contracts/src/mixins/MTransactions.sol

@@ -26,6 +26,9 @@ import "../interfaces/ITransactions.sol";
 contract MTransactions is
     ITransactions
 {
+    // TransactionExecution event is emitted when a ZeroExTransaction is executed.
+    event TransactionExecution(bytes32 indexed transactionHash);
+
     /// @dev Executes an Exchange method call in the context of signer.
     /// @param transaction 0x transaction containing salt, signerAddress, and data.
     /// @param signature Proof that transaction has been signed by signer.