Strip leading / from key names when listing bucket

Fixes an issue with #2404 discovered by @kingb by making sure that key
names do not start with a leading `/` when calling list on a bucket.
This fixes the incorrect storage class being set when you fetch a key
like this:

```python
b = s3.get_bucket('foo-bucket')
k = b.get_key('/some/path.txt')
print(k.storage_class)
```

cc @jamesls, @kyleknap