Allow to specify remote hostname even for the Gmail case

Previously we hard-coded the imap server name in the case of Gmail
repositories, but often we need a different host name. So, allow people
to specify the hostname via the regular "remotehosteval" and
"remotehost" settings, and only falling back to imap.gmail.com when
nothing has been specified.

Cache the hostname, so we don't evaluate the whole thing each time we
query the host name.

Make the remotehosteval processing more robust, by catching any
Exceptions that occur, and throw a OfflineImapError, that explains where
exactly the error had occured. You can test this, e.g. by setting
remotehosteval to 1/"n" or some other invalid expression.

The whole IMAP.gethost() function has been documented code wise while
going through.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Base.py

@@ -21,7 +21,7 @@ import traceback
 from offlineimap import CustomConfig
 from offlineimap.ui import getglobalui
 
-class BaseRepository(CustomConfig.ConfigHelperMixin):
+class BaseRepository(object, CustomConfig.ConfigHelperMixin):
     def __init__(self, reposname, account):
         self.ui = getglobalui()
         self.account = account