Add events for receiving and sending

[ligi]

As discussed with @rmeissner at the demo day before Devcon - opening this as an issue:

I understand that the events where not added to save gas-costs. But the costs are now just there in another form: the form of centralization and the need to run tracing servers. So I would love to open the discussion to change this. At least for receiving there should be an event thrown. IMHO the costs would not be significant and the benefits would outweigh the costs. Let me know what you think!

context:
https://gitter.im/gnosis/Safe?at=5d8c7a97290b8c354ad93891
coder5876/simple-multisig#41 (comment)

[mpetrunic]

👍

[rmeissner]

@ligi @mpetrunic could you comment on the PR what parameters you would like to see and what information should be indexed.

[mpetrunic]

Something like TxInitiated, TxConfirmed and TxExecuted with address of actor so we can setup notifications with some existing service like Tenderly when something happens. Instead of pinging team members and checking safe website every morning. I don't think I need anything indexed.

[rmeissner]

See #135 for all the events.