Moved elbows out from being defined in attributes in columns to instead being defined as an element (<lcars-shoulder>) with an attribute for shape (shape="...") which can be round or square. First column in index.html and all of elbows-demo.html upgraded to show this new usage.

Important to note:
 - This required using jQuery. Unfortunately Zepto does not implement the "nextUntil" function on selectors which was required for the changes in the columns. Obvious solution is to submit a PR to Zepto!
 - Not yet implemented column with just a single elbow. Next commit!

Signed-off-by: Martin Evans <martindevans@gmail.com>