Add settings.BOTO_S3_KEY_PREFIX

[radeksimko]

As number of buckets per account is limited, I believe many people will strive to use prefixes and keep related content in the same bucket.

Therefore I think an option for setting a prefix which can be used throughout the app might be handy.

Then you wouldn't have to call all the S3Storage methods (delete, exists, listdir, size, url ...) with concatenated name, but there might be just a helper for the key name.

I can send a pull request for this, but before I do that is there any idea why not doing that - any feedback?