Update wpseo-functions.php

The URL to ping Bing is incorrect. I've updated the the URL to the correct one.
Prevent from loading if WP is installing
Add filter to allow other plugins to interact with out metaboxes outside of the standard pages
Renamed the filter and added API documentation
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1242-metabox-loading
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Prevent-from-running-on-wp-install-when-in-mu
Replace variables through an AJAX call, which makes them work in the post editor too and allows for more variables to be replaced in the title.
Preload `%%sep%%` for performance reasons. Fixes #1260 (together with previous commit)
Merge pull request #1275 from jrfnl/Issue-1242-metabox-loading

Add filter to allow other plugins to interact with our metaboxes outside...
Merge pull request #1271 from jrfnl/Prevent-from-running-on-wp-install-when-in-mu

Prevent from loading if WP is installing
Merge pull request #1268 from GodThor/patch-1

Update wpseo-functions.php, fixes wrong bing ping URL, props @GodThor