Change container divs from IDs to classes

#wrapper and #container should be made into .wrapper and .container, so they can be easily reused throughout the page while avoiding xhtml restrictions on one ID per page. (Not necessarily germane to HTML5, but something that would not hurt.)

For those wondering why this might be useful: This is a way to define multiple squaregrid grids on a page without having to copy and rename grid width values over and over. One might want to define multiple squaregrid grids on a page to facilitate interesting page background strategies, for example.

Consideration: Don't want this to break existing sites that may have targeted these div IDs in child theme CSS.