Adding filter for stopwords

* Added stopwords filter including filter documentation
* Checked functions using the stopwords filter if they'd be able to deal with an empty stopwords array and fixed if needed
* Came across potential bug in one of these functions and fixed that (class-admin.php, line 10)

This addition resolves a number of issues reported on the forum, amongst others:
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-disable-post-slug-stop-word-modification
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-re-enable-stopwords
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-stop-words-in-spanish