Batch broadcast sending

Here's a pass at batch broadcast sending with the Mailgun api. It doesn't support more than 1000 emails at once, but hopefully Mailgun will fix the issue I reported before we get there :).
`JSON.dump` has something to do with marshaling and unmarshaling (here's the last sentence of the dump docs):

> This method is part of the implementation of the load/dump interface of Marshal and YAML.

I couldn't reproduce the actual loading and dumping of arbitrary Ruby objects (maybe this has been eliminated, I vaguely remember a security issue in the past about this), but I figure that we should use the method that's meant for what we want, which is generate, even if they superficially seem to do the same thing.