Theme Installer: Revert to proxying through PHP for WordPress.org API requests.

This is to ensure we have valid installation nonces, though we've run into this as a problem previously (see #27639, #27581, #27055).

A tad slower, but we gained speed in 3.9 by simplifying the request made to the API.

props ocean90.
fixes #27798.

Built from https://develop.svn.wordpress.org/trunk@28126


git-svn-id: http://core.svn.wordpress.org/trunk@27957 1a063a9b-81f0-0310-95a4-ce76da25c4cd