adding the possibility to change disk config at runtime through middlewares and support for s3 private buckets

[takifouhal]

couple of changes that allow changing root path or any disk config at runtime through middlewares based on demand (user id for instance). adding signed temporaryUrl instead to url for s3 adapter to tackle the problem of private buckets.

[wi-wissen]

Changing disk config at runtime works perfectly. 🙂 Thanks!