Closes #133: Added events + required refactoring

[rmeissner]

Closes #133

This PR adds events for all kinds of interactions with the Safe. This will make it easier for 3rd party clients to monitor Safe instances.

The additional costs are between 2% when executing a transaction. For incoming transactions there is an additional 1k required.

Current event definitions (for a better overview):

    event ApproveHash(
        bytes32 indexed approvedHash,
        address indexed owner
    );
    event SignMsg(
        bytes32 indexed msgHash
    );
    event ExecutionFailure(
        bytes32 txHash, uint256 payment
    );
    event ExecutionSuccess(
        bytes32 txHash, uint256 payment
    );

    event ExecutionFromModuleSuccess(
        address indexed module
    );
    event ExecutionFromModuleFailure(
        address indexed module
    );

    event IncomingTransaction(
        address from, uint256 value
    );

[rmeissner]

Alternative to the Execution and ExecutionPayment events (to save some gas):

• Execution(bytes32 txHash, bool success, uint256 payment)
  • Transaction information can be parsed from the transaction where the event was thrown
  • Only edge case is when an other contract uses an internal transaction where the data is not in the transaction meta (e.g. when using the original Gnosis MultiSig contract)
  • It could be checked if encoding these values packed safes more gas

[ligi]

Thanks for the fast PR to address #133 - event looks good to me - hope it will get merged.

[rmeissner]

updated events for gas efficiency: transaction details have been removed from events, only the transaction hash and the payment are part of the event, the rest needs to be retrieved from other sources (e.g. the Ethereum transaction or our service).

[Uxio0]

Looks good to me