Refactor logging

This pull request adds logging for all incoming requests, clarifies existing log levels, and adds a `HekaLogger` implementation for compatibility with the Cloud Services [logging standard](https://mana.mozilla.org/wiki/display/CLOUDSERVICES/Logging+Standard).

* HekaLogger` logs Protocol Buffer (`HekaProtobuf`) or JSON (`HekaJSON`)-encoded log messages to standard output.
* `TextLogger` replaces `StdOutLogger` for logging output using @jrconlin's human-readable format (see jrconlin/FindMyDevice@65c0f33684e4d01a9e70d02d497d17668aa2fe3b).
* All incoming requests and `Worker`s are tagged with the `X-Request-Id` header. This is a convention used by [Heroku](https://devcenter.heroku.com/articles/http-request-id), [restify](http://mcavage.me/node-restify/), etc., and allows correlating log lines for the same request. This value is sent in the `t` log field.
* `LogHandler` wraps `http.Handler`s and logs request replies. `logResponseWriter` wraps `http.ResponseWriter`s to record response summaries.
* Add an `ErrorCode` type and application error codes. These are not currently logged or forwarded to the client, though I think it would be nice to do both soon.
* Logger calls that may incur overhead (such as function calls) are now wrapped in `Logger.ShouldLog()` checks. This check is omitted if the log fields contain simple string values, since `Logger.Log()` calls `Logger.ShouldLog()` internally.
* Remove the last few `log.Printf()` and `debug.PrintStack()` calls.
* `Logger.Log()` and all `SimpleLogger` convenience methods now return errors, and use syslog severity levels for compatibility with the Heka message format.
These correspond to syslog severity levels for the Heka message envelope, though we only ever use critical, error, warning, info, and debug. Should we remove the others?