logging: add tracing to log calls to aid in debugging

[admtnnr]

In the martian.Context for each http.Request is a short lived unique(ish) ID. It would be helpful for debugging to be able to include this "trace ID" in every log line that occurs to be able to follow the execution/modification of a request/response.

My initial idea is that a package creates a Logger instance that it holds and passes the martian.Context to that will obtain the information it needs and reformat as appropriate.

package mypkg

var log = martian.NewLogger("mypkg.Modifier")

type Modifier struct {
  ...
}

func (m *Modifier) ModifyRequest(req  *http.Request) error {
  ctx := martian.NewContext(req)

  // Logs with ctx.ID(), the package and the formatted log line. 
  log.Debugf(ctx, "modifying request: %s", req.URL)

  ...

  return nil
}