Fix country-specific GMail folder names

 * Germany (and Austria) now use GMail as the base folder names.
 * Suggests users to check their spam and trash folder names
   and set them appropriately.

From: Bart Kerkvliet <bkerkvliet@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Refactored authentication handling

 - created helper routine that will do authentication;

 - routine tries each method in turn, first successful
   one terminates it: makes things easier to read
   and handle;

 - renamed plainauth() inside offlineimap/imapserver.py
   to loginauth(): the function does IMAP LOGIN authentication
   and there is PLAIN SASL method, so previous name was
   a bit misleading;

 - slightly improved error reporting: all exceptions during
   authentication will be reported at the end of the run;

 - now loginauth() is never called if LOGINDISABLED is advertized
   by the server; it used to be invoked unconditionally when
   CRAM-MD5 fails, but we should respect server's opinion on
   how to handle its users.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>