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.

Take a look at this (idea is to allow using the Heka config to manipulate the message via Toml configuration, e.g. adding the DSN to a sentry message later than the decoder, or populating values from some fields into other fields, as a PayloadRegexDecoder can do). 

https://github.com/Clever/heka-clever-plugins/pull/9

Do you think this is beyond what an Encoder should do to modify the message? Would this make more sense as a Filter?