Verifying Facebook's signed_request in Ruby

Facebook's signed_request parameter can be quite complicated to parse in Ruby. Facebook's examples are of course entirely in PHP. signed_request is their new way of delivering data to your app instead of individual fb_sig_ parameters for everything. Here is the code to properly verify the signed_request parameter, and return a hash with all of the data from the request. Just call parse_signed_request passing the received params from the HTTP request, and your app's secret key (issued by Facebook). An exception will be thrown if verification fails, otherwise, you'll get a hash of the data back.