Add the target-confirmations parameter to listsinceblock

Fixes #57
> According to [Bitcoin wiki](https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list):
>
> > listsinceblock	 [blockhash] [target-confirmations]
> > Get all transactions in blocks since block [blockhash], or all transactions if omitted. [target-confirmations] intentionally does not affect the list of returned transactions, but only affects the returned "lastblock" value.
>
> Can be fixed with a two-line change in `connection.py`.