Support for versioning and prefixing

[virtosubogdan]

There should be an option to augment the prefix for static files.
This should be done by a static prefix and a dynamic option (for versioning).

The use case for the static prefix is that a bucket contains a lot of resources for many applications/environments.
It seems similar to #22, but because the static file location does not depend at all on the application logic, this is completely different.
The static files should be collected to /path_to_allowed_prefix/environment/static_files on the S3 bucket, where /path_to_allowed_prefix/environment/ should be specified by a setting.

The use case for versioning is the standard one. Appending a hash to the filename is an alternative, but the choice between hash appending and another prefix should be up to the flask-s3 user.
The proposition is to do this by another setting which will contain the path to the function where the user implements the versioning prefix.

All requests should add no overhead when collecting or using url_for for users that do not want to use them.

[Fuyukai]

Closed by #64