Issue 100 - Post Achievement badges to Facebook

The 'Share' button will show for newly created achievements with a Facebook object ID and users who signed in with Facebook. 

A script is needed to generate Facebook object IDs for previously created achievements.

The following edit needs to be made to `config/initializers/devise.rb`. The extra part is the :scope parameter requesting publish_actions.

`config.omniauth :facebook, "<app-id>", '<app-secret>', { :scope => "publish_actions" }`

Existing Facebook users will be asked for the additional permission when they next sign in.

This whole feature won't actually work until the Facebook app passes review, but it needs to be pushed so the reviewer can see the feature in action.