Front-end should use home_url() instead of site_url()

See difference between [home_url()](http://codex.wordpress.org/Function_Reference/home_url) and [site_url()](http://codex.wordpress.org/Function_Reference/site_url), including some discussion on [this StackExchange question](http://wordpress.stackexchange.com/questions/20294/whats-the-difference-between-home-url-and-site-url). This is important when [giving WordPress its own directory](http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory) as site_url() will point to the WordPress install directory, with home_url() pointing to the front-end. Currently this plugin doesn't work on our sites that have WordPress in its own directory, since it uses site_url() for everything and WordPress doesn't know what do with these URLs.