Modified new layout and stylesheet file naming to enforce best practices
around theme structure. The new rules are:

For layouts:

* `name` -> `layouts/name/name.html`
* `theme/name` -> `layouts/theme/name.html`
* `theme/subfolder/name` -> layouts/theme/subfolder/name.html`

For stylesheets:

* `name` -> `layouts/name/style.css`
* `theme/name` -> `layouts/theme/name.css`
* `theme/subfolder/name` -> layouts/theme/subfolder/name.css`

The edit handlers can still modify styles in `css` and the base `layouts`
folder, for compatibility on existing sites. This ensures all new themes
are structured according to best practices automatically.
Layouts in subfolders of themes should still be editable
Merge branch 'master' of github.com:jbroadway/elefant