TypeError: string indices must be integers

Hi,

Getting errors on the batch example. This is in Python 2.7.

```
>>> commands = [ [ "getblockhash", height] for height in range(100) ]
>>> block_hashes = rpc_connection.batch_(commands)
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python2.7/dist-packages/bitcoinrpc/authproxy.py", line 169, in batch_
if response['error'] is not None:
TypeError: string indices must be integers

```

Start with authproxy.py at line 169, where batch_ handles the responses shown in the traceback. Reproduce the Python 2.7 batch example with getblockhash commands and inspect the returned response shape. Done means the example completes without the string-indices TypeError and returns the expected block hashes.

Scritto dal modello di indicizzazione a partire dal testo della issue.