require encoder for SentryOutput

As a best practice, I'm converting outputs to use encoders. Setting `PayloadEncoder` keeps the previous behavior for all Outputs I've modified this way, so far.

This lets us also use a FieldEncoder to remap fields via configuration in Heka toml: https://github.com/Clever/heka-clever-plugins/pull/9

Most immediately I want to use this to write the SentryDSN via FieldEncoder when the message gets to the `SentryOutput`, instead of needing to have the SentryDSN written during the decoding step, which we're doing now.

Yeah, I don't think `Encoder`s are supposed to mutate the message. I think they have a strict job of knowing how to take a message and extract some information from it. Any sort of modification should be a `Filter`, I think. What do you think?