Allow to define a path prefix in S3DataStore

I've been working with a S3 bucket where lots of different contents are uploaded in different ways, so at some point it became a real mess because of all the files sharing the "root" namespace. Setting each time a custom path is possible but it enforces you to build your own naming mechanism which is an overkill for a simple file upload.

One logical solution was to preset a path prefix, so that all files which are uploaded from inside a form will be saved using a path convention, such as "attachments".

This feature is totally optional, the S3DataStore will work without making any changes to the old code. Just set the path_prefix option to a desired value and you are good to go.