support doubleclick to display advertising

using doubleclick.net script instead of google-analytics.

see it: https://support.google.com/analytics/answer/2444872?hl=en&utm_id=ad

Usage:

use Rack::GoogleAnalytics, :tracker => 'UA-xxxxx-x', :advertising => true
travis build status
Merge pull request #27 from masarakki/enable_doubleclick

support doubleclick to display advertising
fix tests. i wonder if checking for the content length is so useful ;)