Patch from Laurent Petit lpetit@sqli.com to add smtp authentication functionality (massaged by me)

• config_defaults_inc.php
  (smtp_username): new option, giving username for smtp server when using
  phpMailer
  (smtp_password): new option, giving password for smtp server when using
  phpMailer

• core/email_api.php
  (email_send): use the new options

• doc/configuration.html: document the two new options

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1807 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9