Added support for ticket based authentication.

==Notes==
Perforce servers configured to security level 3 do not accept passwords sent using the -P parameter of p4 commands. Instead they expect a ticket that is generated when running the "p4 login" command. For more information on this Perforce feature, see http://www.perforce.com/perforce/doc.current/manuals/p4sag/03_superuser.html#1080319.
This changes add a new user setting (defaulting to off) that enables ticket-based authentication. When enabled, the plugin will run "p4 login -p" to obtain a valid ticket it can use in lieu of the user password when issuing commands.
Improved error message when authentication fails and users did set their credentials.
Merge pull request #1 from agersant/ticketAuth

Ticket-based Perforce authentication