Optional verbose debug logging for http requests

[cbroglie]

No description provided.

[huandu]

@cbroglie Per #28, I plan to implement a debug mode with Facebook debugging API. I'll use Session#Debug for this purpose and put all http headers into the Result returned by Api/Get/Post/Delete. So I cannot merge your PR, but I'll consider to implement similar feature like yours in my code.

Apologize for closing this PR without merging, and thanks for your contribution all the same. :)

[cbroglie]

Cool, it sounds like #28 will cover my needs

[huandu]

@cbroglie I've implemented debug mode in b7c0cf9. You can take a look and play with latest master code if you like.

When debug mode is turned on by setting fb.Debug = fb.DEBUG_ALL or session.SetDebug(fb.DEBUG_ALL), you can get DebugInfo from any graph api result. See DebugInfo and Result#DebugInfo document for details.