adding .col-md-* to desktops >940 & moving .col-lg-* to  desktops >1200

adding .col-md-* to desktops >940 & moving .col-lg-* to  desktops >1200

there's been discussion of doing this, but i havent seen any code submissions for it... so I'm submitting these proposed changes. this is my first 'pull request' so i apologize if i'm doing this incorrectly or in the wrong place.