Rework menu navigation: these changes build off pull request #239 for issue #200
 * Only set button and menu if container is not null.
 * Break up the "return early" conditional into multiple parts.
 * Shorten the name of the toggled class from `toggled-on` to `toggled`.
 * Restore the "Hide menu toggle button if menu is empty." documentation.
 * Remove the `nav-menu` class. The previous code would replace any custom custom classes applied via `wp_nav_menu()` or template file.
 * Replace the `nav-menu` selector is style.css with selectors that will target both a custom nav menu and a page menu. These changes could use critical feedback. Do you know of an edge case where they might not work as expected?
Rework Navigation Menu Part 2
 * Separate scoping and assignment so that a decision does not need to be made as to whether we should intermix spaces and tabs on the left
 * Restore the previously removed nav-menu class addition to the first unordered list in `#site-navigation`.
 * Restore the `.navigation-main.toggled-on .nav-menu` selector.
 * Remove rules for the `.main-small-navigation ul` selector.
Merge pull request #242 from mfields/navigation-menu

Rework Navigation Menu. Closes #200, #205, #237, #243.