Updating core WordPress from 3.8.1 to 3.9.1

* 3.8.1: https://github.com/WordPress/WordPress/commits/831b3ca50dd98fdd
* 3.9.1: https://github.com/WordPress/WordPress/commits/3.9-branch
* 3.9.1: https://github.com/WordPress/WordPress/commit/c462d023ddf12654f
* Built from https://develop.svn.wordpress.org/branches/3.9@28353
* By @nacin on 8 May.