Formalize how and where we save the files.

[ehgoodenough]

It would be great if we could create a module that saves the files. The interface for the module should accept a file write stream, and then save it somewhere. We can create two different modules that satisfy this same interface, one to save to a directory on the local file system, while the other saves to an S3. Even better, if no environment variables are on the machine for the AWS credentials for the S3, it would default to the local file system.