Handle bytes content without encoding; add regression test.

Resubmission of https://github.com/seomoz/reppy/pull/14, rebased off master.
This seems to be another python 3-specific issue. To reproduce, open python 3.x, then:
```python
> from reppy import cache
> robots = cache.RobotsCache()
> robots.allowed('http://google.com/', agent='robot')
...
  File "/Users/jmcarp/code/reppy/reppy/cache.py", line 66, in cache
    fetched = self.fetch(url, *args, **kwargs)
  File "/Users/jmcarp/code/reppy/reppy/cache.py", line 84, in fetch
    raise exceptions.ServerError(exc)
reppy.exceptions.ServerError: Type str doesn't support the buffer API
```
So `fetch` will fail on any `robots.txt` file that doesn't start with a BOM character, which is probably most of them. Without sniffing the encoding, not sure how best to handle this. What do you all think?