Small improvements

Main one is switching to mailcore2

Notes: MailCore2 [might not be 10.7 compatible](https://github.com/MailCore/mailcore2/issues/566#issuecomment-34891672) so if this is OK we should probably make the plugin 10.8+ only
I noticed a warning that `tls` isn’t used any more, which is true. I feel like an explicit setting in the client should carry some weight though. Maybe the `default` for the `switch` statement should change to

    connType = tls ? MCOConnectionTypeTLS : MCOConnectionTypeClear;