Rework cookie persistence handling
Rework cookie persistence handling

Instead of trying to look for a specific cookie to save, discarding all
others, and persisting the cookies manually using pickle, we build on
the functionality of cookielib, which already has functionality to save
and load cookiejars. The request library is documented to work with any
subclass of cookielib.CookieJar.

This ensures that we only save persistent cookies (which includes the
X-APPLE-WEB-KB cookie), and skip session cookies, which should make the
code more future proof in case Apple adds more persistent cookies.

This also fixes #44, which was still occurring because we were
persisting the cookies of the request, not the session, and when
logging in with a persisted cookie the resulting request did not
have the X-APPLE-WEB-KB set, so we ended up overwriting the cookie
file with one that didn't contain any X-APPLE-WEB-KB cookie anymore.

picklepete/pyicloud,picklepete/pyicloud