Add `make proto`.
Replace `make proto` with the `message` package.
`main.VERSION` => `simplepush.VERSION`.
Add Heka logging support.

* `HekaLogger` logs Protocol Buffer (`HekaProtobuf`) or JSON
  (`HekaJSON`)-encoded log messages to standard output.
* Replace `StdOutLogger` with `TextLogger`. The latter logs simplified,
  human-readable log messages to standard error.
* `Logger.Log()` and all `SimpleLogger` convenience methods now return
  errors.
* Use syslog severity levels for compatibility with the Heka message
  format.