Refactoring and cleanup.

Extract post-specific logic in bu_navigation_get_urls() in to separate functions
Switch to static variables for caching in bu_navigation_get_page_uri()
Don't return absolute URLs if ancestors aren't found in bu_navigation_get_page_uri()
Adding doc blocks for new functions.
Cleanup in `_bu_navigation_page_uri_ancestors`.

Use late priority for field filter to ensure it wins.
Comment rewrite.
Formatting cleanup.

Spaces to tabs for bu-navigation-labels.php.
Fixed bad method name in navigation test case.

Formatting.
Test coverage for new library permalink functions.

- Ensure our functions return the same results as the functions `get_permalink()` uses.
- Adjustments to library functions to fix issues uncovered by said test cases.