Http client fixes

This two patches allow go-pushbullet to work on appengine.

The first allow injecting a custom http.Client, which for appengine must be from the appengine/urlfetcher package instead.

The second patch is a bit more invasive.  The appengine SDK doesn't like urls with the embedded username, but that's how the http requests were being constructed.  Instead, we now explicitly create http.Request objects and set the headers and body fields by hand.