Add limit support to S3 API

[lukemarsden]

#686 hard-codes the maximum number of files that can be returned by the dotmesh S3 API to 100, we should make the limit (and start, for pagination) configurable on the request instead, and also mark the response with some value which indicates when there's files missing in the response probably.

[Karolis Rusenas]

Need to add pagination or just completely refactor how we present files.