Adding a fix to account for empty "/" keys added by the console.

- Using a filtered iterator that ignores "/" suffixed keys.
- See: https://forums.aws.amazon.com/thread.jspa?threadID=147981&tstart=0
- Adding integration test
Merge pull request #257 from aws/stream_wrapper_dir_fix

Adding a fix to account for empty "/" keys added by the console.
Merge remote-tracking branch 'miklos-martin/master' into stream_wrapper_mkdir
Updating S3 stream wrapper to support console style sub-folders.

- Moving creating buckets and pseudo folders to different methods.
- Making mkdir checks more explicit by checking (vs using is_dir).
- Fixing tests and adding an integration test for mkdir() with a key.