Ignore EISDIR errors while scanning for files in storage

Fixes #10.
I know the saying “it's easier to ask for forgiveness than permission” but as soon as you start playing with errno and so on, the code becomes hard to read IMHO.

Why not:
```python
if os.path.isdir(full_filename):
     continue
with storage.open(full_filename) as f:
     ...
```