Utility for read events, and helper method in `rpc.TransactionReceiptWithBlockInfo` using it

A utility to filter events in a `[]rpc.Event` slice would be nice to have, and have a method in the `rpc.TransactionReceiptWithBlockInfo` type to filter the events from it would also be great.

Ex:
``` go
myEvent := utils.EventWith(eventSlice, "event_key")
```

``` go
myEvent := txReceipt.EventWith("event_key")
```