(optionally?) use s3 to store bundles?

Related to #138. Bundles can be kinda big. It might be a good idea to store them on s3 instead of redis or leveldb, but it's hard to say.

Having this would largely necessitate the sort of pluggable backend mentioned in #138.

Start by reviewing related issue #138 and how bundles are currently stored in Redis or LevelDB. Determine whether an S3-backed option and the proposed pluggable backend have a defined scope; the work is complete only once a storage direction and implementation requirements are agreed.

Written by the indexing model from the issue text.