Factor loader into a database storer and a batch loader

I was on the fence about this, but according to https://golang.org/doc/effective_go.html#generality I should return the interface and not my class.

Mocking the bucket kind of sucks without an interface, I'm with you, but I don't want to make every caller of this class connect to a bucket and pass it in. I might make an interface, split out a helper method, and make this method a thin wrapper that just connects to the s3 bucket.