Added support for repository "syncdate" option so email's date are used for local file timestamp.

This commit adds support for a synchronizing message's date with local file time (when downloading the message or retrieving them from another Maildir), when the "syncdate" option is set to True in the config's "Repository" section.

This is useful when you first run OfflineImap on a folder with thousand of messages, because without the options, they will have "now" timestamp. You can't use any UNIX tools from that (for example, you can't call find to find messages which are older than 2 days, etc...)

If your Maildir is used by an IMAP server for external access, then the messages' timestamp will be shown wrong too.

This is also useful if you run OfflineImap sparsely (for example, for backup once a week).