Added support for authorization using client-side certificate. It's suporting POP3 and IMAP protocols.

K-9 Mail supports authorization with client-side certificate using IMAP protocol. This feature is available for all Android systems since 4.0 version (Ice cream sandwich). In order to set up account to use that kind of method you need to:

1. Add new account from accounts screen,

2. Insert correct email and password in login screen, press Next,

3. Choose IMAP or POP3 protocol,

4. In incoming server settings set security level to “SSL(always)” and insert correct data supplied by your email provider,

5. Application will try to connect to your email server and if prompted, it will show dialog asking for client-side certificate. You need to have installed certificates on your device. If you didn’t install them yet, then application will ask you if you want to do that now and if confirmed application will bring you to Android security settings, where in most version you need to tap option called “Install certificate from phone storage”  in “Credential storage” group. System will automatically detect all certificates stored in your phone and it will let you to install them. Now go back to K-9 Mail and choose appropriate certificate.

6. In outgoing server settings you also need to set security level to “SSL(always)” and insert correct data supplied by your email provider. If needed, server will use the same certificate for sending email as the one for fetching them. 

7. Server will check if settings are correct and if they are then we are done!