Option to generate a random password on user creation/update.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11456 e93f8b46-1217-0410-a6f0-8f06a7374b81
Use User#generate_password in MailHandler.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11457 e93f8b46-1217-0410-a6f0-8f06a7374b81