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?
I'll add a comment—it's the same trick for recycling the slice used by `marshalFrame`. `m.Uuid[:0]` sets the length to zero, but preserves the capacity of the backing array (i.e., `len(m.Uuid) == 0 && cap(m.Uuid) == 16`). That way, `id.Append()` can just reset the length (`m.Uuid[:16]`) instead of allocating a new slice.