Refactor single.php, avoid PHP notices when parts are not set

* esc_url() on output of featured image URL
* absint() on position pulled from post meta
* Use get_the_ID() rather than $post->ID
* Add some documentation
Add checks for data, escaping in parts/headers.php

* esc_attr() around site name and tagline
* esc_html() around site name and tagline
* Check for category existence before attempting to output
* Remove is_category and is_archive checks.
Add is_archive and is_category back to parts/headers
Merge pull request #10 from washingtonstateuniversity/clean-spine

[PR] Cleanup of single.php and parts/header.php. Thanks, Felt. I know this parent theme needs a LOT more tlc.