Merge _s[cfda009] : Better WordPress title handling for WordPress 4.1+ with proper back compat.

Better WordPress title handling for WordPress 4.1+ with proper back compat.
see. https://github.com/Automattic/_s/commit/cfda0096b3700be474fad9011ae8056ec7ead723
* Adjust todo notice for wp_title shim.
* Better title handling for WordPress 4.1. See #644.
* First pass at better title handling for WordPress 4.1+ with proper back compatibility baked in.