Add `make gen`. Check in Heka log message schema.

`make build` now installs binaries for pinned dependencies.
Remove the Heka client dependency. Closes #125.

* Add log message convenience functions from Heka.
* Add `id.GenerateInto()`.
* Remove `LogFields.Names()`.
* Remove `Heka{Emitter, Sender}`.
`id.GenerateInto()` => `id.Append()`.
Add basic log emitter tests.
Remove `id.Append()`.

Benchmarks: 1510 ns/op for `GenerateBytes()`, compared to 1425 ns/op
for `Append()`. `newHekaMessage()` with `GenerateBytes()` took
3765 ns/op and 16 allocs/op, compared to 3515 ns/op and 15 allocs/op
for `Append()`.
Clean up log emitter initialization.

Clarify `TryClose` behavior.
Merge pull request #161 from mozilla-services/feature/remove-heka

Remove Heka client