Enable / Disable option to force logout functionality

I put an option in plugin to Enable / Disable option to force logout functionality. Like, shown in below:
![force-logout](https://cloud.githubusercontent.com/assets/2002921/3208440/78a7a85c-ee34-11e3-94b8-e95f7b4c1522.png)

It would be helpful to those who need this functionality. By default, force logout will be set to Yes. If it is set to No then user will remains logged in all other session.

By Default it will be 'Yes/1' so it will not affect Backward Compatibility.

Issue Tracker Link:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33828
Thanks for review. I will update it to "Set to No to disable this". :)