Enable API debug output by setting HUB_VERBOSE

This will print information about every HTTP request and response to stderr. Authentication info is redacted.

    $ HUB_VERBOSE=1 hub ci-status
    > GET https://api.github.com/...
    > ...

This option for debugging is crucial for some of our users that submit reports about API failures, but their console output is not enough to debug their issue.

@jingweno: Would it be possible to get the same feature in Octokit?