Ensure that Subtitles works in PHP 5.2+.

The first three versions of Subtitles relied on at least PHP 5.3. Unfortunately, late static binding isn't available to PHP 5.2.4, which WordPress supports as a minimum requirement and which is currently used by over 42 percent of all WordPress installs. http://wordpress.org/about/stats/

This change adds the Singleton design pattern into Subtitles_Admin and removes any functions that are specific to PHP 5.3 only. See https://github.com/philiparthurmoore/Subtitles/issues/8.