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.

The dependencies have changed, so this will require a `make clean` before building locally. Closes #125.

@jrconlin, @bbangert r?
This feels like a premature optimization. One thing to remember, is that if you create something in a Go function, and it never escapes the scope of the function, it stays on the stack so its immediately collected when the function exits. No GC needed. As such, I don't understand why GenerateBytes couldn't just do this uuid generation into a new slice, then use the normal append method.