Fix #26: expose bucket iterator w extra params

Presumes #358 is landed first.

Exposes additional paramters for narrowing / batching key search.

Fixes #26.
Would it be confusing if we used

```python
def iterator(self, **kwargs):
    ...
```
and then just checked each key against a translation dict:
```python
{
    'prefix': 'prefix',
    'delimiter': 'delimiter',
    'max_results': 'maxResults',
    'versions': 'versions',
}
```