update social plugins for July 2013 values

Facebook added support for the [kid_directed_site social plugin and JavaScript SDK parameter](https://developers.facebook.com/docs/plugins/restrictions/) to limit collected data in a privacy-sensitive environment such as a website targeting children under the age of 13 in the United States. Add a new checkbox to the Facebook general settings page to limit available social plugins and initialize the Facebook SDK for JavaScript with a `kidDirectedSite` value of `true`.

Remove the border_color option from social plugin boxes. Sites previously relying on this parameter should instead directly style the elements using CSS.

Remove the social plugins CSS file bundled with the plugin and loaded with each requests. Sites should apply their own styling if needed.

Update social plugin parameter setters to explicitly cast parameter values to appropriate types such as `int` and `bool`. Improve handling of parameters containing underscores.