Better type errors

In 0.9.2 (and master), if one passes a `group` to `SimpleConsumer` that is a unicode string, it bombs with the terribly unhelpful message below. This PR merely makes the error more helpful by providing a message describing what should have happened and what type was provided. With it in place, the fix would be obvious.
```
Traceback (most recent call last):
  File "sniff_topic.py", line 52, in <module>
    main(sys.argv)                          
  File "sniff_topic.py", line 38, in main   
    cons = SimpleConsumer(client, group, topic)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/consumer.py", line 243, in __init__
    auto_commit_every_t=auto_commit_every_t)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/consumer.py", line 115, in __init__
    fail_on_error=False)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/client.py", line 384, in send_offset_fetch_request
    resps = self._send_broker_aware_request(payloads, encoder, decoder)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/client.py", line 151, in _send_broker_aware_request
    correlation_id=requestId, payloads=payloads)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/protocol.py", line 482, in encode_offset_fetch_request
    message += write_short_string(group)
  File "/home/ubuntu/venv/local/lib/python2.7/site-packages/kafka/util.py", line 23, in write_short_string
    return struct.pack('>h%ds' % len(s), len(s), s)
struct.error: argument for 's' must be a string
```

p.s. If you have not yet tried [PyCharm](https://www.jetbrains.com/pycharm/) you don't know what you're missing. I'm surprised that unittest2 allows such things, but the `test.test_util.UtilTest` class redeclared the same method name over and over.
That's a funny thing about Python objects, you can set any attribute you'd like without much complaint. As for this particular assignment, if effective does nothing but create an attribute in the object's `__dict__` with a value of None, so it's pretty low impact.

I do believe it is vestigial code though, I don't see any other references to "timer" anywhere.