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.

I thought the `Encode` method returned a byte slice that is the encoded version of the message? That's what https://hekad.readthedocs.org/en/latest/developing/plugin.html#encoders implies. So, for instance, I believe that `PayloadEncoders` return the `Payload` field, which is a byte slice.