Add optional support for google display advertising tracking to analytics.

Display advertising lets sites turn on interest and demographic reports as well
as leverage marketing and display network impression tracking.
https://support.google.com/analytics/answer/2444872

This is off by default as it requires privacy policy changes to turn on.
https://support.google.com/analytics/answer/2700409
Update docs to use a simplier version of the support url.
consistent spacing for the analytics config definitions.
Merge pull request #42 from isaacgrant/display-network-analytics

Add optional support for google display advertising tracking to analytics.