TST: s3 codepath in get_filepath_or_buffer isn't tested.
This codepath (I think) does not get tested in the test suite. (if you add a `raise AssertionError("I SHOULD FAIL")`, after `if _is_s3_url(filepath_or_buffer):`, it does not fail).

``` python
    if _is_s3_url(filepath_or_buffer):
        try:
            import boto
        except:
            raise ImportError("boto is required to handle s3 files")
        # Assuming AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY
        # are environment variables
        parsed_url = urlparse.urlparse(filepath_or_buffer)
        conn = boto.connect_s3()
        b = conn.get_bucket(parsed_url.netloc)
        k = boto.s3.key.Key(b)
        k.key = parsed_url.path
        filepath_or_buffer = StringIO(k.get_contents_as_string())
        return filepath_or_buffer, None
```

This is _is_s3_url, which gets tested, but only with non s3 urls.

``` python
def _is_s3_url(url):
    """Check for an s3 url"""
    try:
        return urlparse.urlparse(url).scheme == 's3'
    except:
        return False
```
