Respect Do Not Track header for Segment.io.
Add segment.io testing credentials.
Add segment.io tracking by user id.
Fix segmentio identify functions for anon and auth'd users.
Fix header format for aspen, not django.
Bypass request object modification and check DNT directly from header.
DNT code pushed into util.
Updated privacy policy to mention segment.io.
we don't need a new hook for DoNotTrack
Add mention of DNT to privacy policy.
Merge pull request #2334 from gittip/2334-do-not-track

Support DNT header (and add anonymized tracking)