Remove Heka client

* Check in the Heka log message schema.
* Add log message convenience functions from Heka.
* Add a `make gen` task for generating Protobuf and Cap'n Proto sources.
* `make build` now installs binaries for pinned dependencies.
* Replace `Heka{Emitter, Sender}` and test log emitters.

Closes #125.

@jrconlin, @bbangert r?
`id.Append` is used by `Message.Identify()` to avoid allocating a new slice per message.