Adding support for getblockhash(index).

## Summary

I noticed that your library says

> Provides a Ruby library to the complete Bitcoin JSON-RPC API. Implements all methods listed at https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_Calls_list .

Yet didn't appear to support `getblockhash(index)`. So I forked the library and added it.

Please let me know if you have any questions.

## Donation

I really appreciate the library and donated .01 BTC https://blockchain.info/tx/f0459920f024f5e2b00158fcd78dbec685d8ba6075e6e2ca21001767078efada. Thank you!