[Feature] Persist SC events

[localhuman]

• Add persistence for events generated via execution of smart contracts
• most important event is the transfer event, so that transaction histories can be generated for NEP5 tokens
• other events may be more difficult to reliably serialize, and should be implemented later
• Perhaps add RPC server to serve queryable interface for transfer events. Other option is to have this be a separate project

Current work is at https://github.com/CityOfZion/neo-python/tree/feature-persist-notifications

[localhuman]

implemented in #161