Add option to Modal to force refresh the content.

Add option to Modal to force refresh the content. This option allows modals to be dynamic and multipurpose. For example, you can use the same modal div for the following two links that request different content:

    <a href="/api/pages/user/profile" data-toggle="modal" data-target="#myModal" data-refresh="true">My Profile</a>
    <a href="/api/pages/user/settings" data-toggle="modal" data-target="#myModal" data-refresh="true">Account Settings</a>

This feature can be tested at http://jsfiddle.net/sCsB9/.

(This pull request deprecates pull request https://github.com/twitter/bootstrap/pull/6793.)