picasa importer: remove Run-private HTTPClient

A more complete version of Aaron Boodman's
https://camlistore-review.googlesource.com/#/c/3066/
in removing Transports.

As Aaron pointed out,
(https://groups.google.com/d/msg/camlistore/9OwQKH6z7rY/840oQrRgsGIJ)
it is illegal for oauth.Transport.Transport to ever be another instance
of oauth.Transport.

So this commit tries hard to use Context's HTTPClient everywhere
and make sure to have its Transport an *oauth.Transport, but that's
Transport is not an *oauth.Transport, but the underlying Transport.

Change-Id: I4defb346abfcf3f6c17597fc2dc23a6e536eda37