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

```