sub_folder option for S3 data store

Hi there -- due to various configuration and other situations beyond our control we need to use "sub-folders" in our S3 buckets. At the moment, dragonfly will calculate the UID based on the current time and a random value, then put that directly into the bucket. Supplying a bucket name as "bucket_name/sub_folder" generates some error conditions, so the solution is to add an extra configuration option in order to supply a sub_folder "prefix" that will be appended to the UID if it was specified.

This is a really simple patch/approach. Please let me know if this is the right way to attack this problem, and if I should do anything else with the patch.

Thanks!