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?
It could; I moved it into the `case` body because it's only used for the Heka loggers. This is for `NetworkLogger`, though; line 248 initializes `FileLogger`. I'll consolidate into a helper function, especially now that the emitters use the same signature.