Add `LogEmitter`.

The `LogEmitter` interface is implemented by `TextEmitter` and
`HekaEmitter`. `NetworkLogger`, `FileLogger`, and `StdOutLogger` now
delegate to an underlying emitter that formats and sends log messages.
Avoid pinning `heka/message`.

Because `gpm` runs `go get` and `git checkout` for all dependencies in
parallel, attempting to `go get` a package from the same repo will fail
when `gpm` checks out a specific revision and `go get` runs
`git pull --ff-only`.

As a workaround, we only pin `heka/client`, which depends on
`heka/message`.