Clean up debug code

[nylen]

Make debug a prototype method. This has a couple of advantages:

• It's no longer necessary for the debug code to be at a specific place
  in request.js, see Remove circular dependency from debugging code #1354
• It's easier for helper methods in other files to generate debug
  messages, see Refactor basic, bearer, digest auth logic into separate class #1360

[simov]

@nylen with this changes #1407 we'll be able to add the debug code as you proposed. We can then access it like

request.debug()

inside each method. All of the debug calls are currently commented out in #1407 so you can easily find them.