Integrated jQuery drag code from Codepen example.

http://css-tricks.com/snippets/jquery/draggable-without-jquery-ui/
(Refactoring) Extract var for 'draggable' css class.
Multiple card drag + fix text limit event binding to use .on().
Clear selection sets when a click starts on the canvas.
Decouple card mouseup handling from mousemove.
Enable single card dragging without having to draw selection square first.

Reset selection set when clicking on a card that isn’t in an existing
selection set.
Focus textarea of just-created stickies.
Fix event coordinate references to work in Chrome and FF.
Delete unused styles.
Allow single click to focus on a single card if it was just moved as part of a selection set.

Also, prevent textarea focus on mousedown.