Adding NTLM Authentication to SMTP connection

The phpmailer library used to send emails support NTLM authentication but it was not possible to choose
the authentication method in Joomla. Changes in this commit are :
- Adding a select box for allowing administrator to choose the authentication method (LOGIN, PLAIN or NTLM)
- Adding corresponding label and description messages into language file en-GB.com_config.ini
- Adding script ntml_sasl_client.php in the 'extras' directory of phpmailer (this file was found on the official repository)
- Changing a flag in the ntml_sasl_client in the negotiation request (there were an unknown value instead of the required NTLM Authentication Flag)
- Changing libraries/joomla/factory.php to take into account the added configuration option (SMTP Authentication Method)