Update sample content. Add site metadata.

Site metadata is in nanoc.yaml. When nanoc 4 will
be released, that content should probably be moved
into site.yaml (which does not exist in nanoc 3).
Add head, beginbody, and endbody tags.

These tags can be used in the frontmatter of any
item to load additional partials just before </head>,
just after <body> and just before </body>, respectively.

This is supported by the default layout.
Rename foundation.js => all.js.

This is used to include all JavaScript scripts that
can be loaded before </body>, not only Foundation.