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 :).
Our of curiosity, what's the difference between `JSON.generate` and `JSON.dump`? I read the docs for `generate`, but didn't understand the difference except that `generate` accepts a bunch of additional options.