Merge pull request #223 from attribution/master

Attribution: Track Page Views
Release 1.2.13
Adding flag for double opt in
Pick up context data for Mixpanel
mixpanel: reject null fields, update
Release 1.2.14
proxy context.screen to resolution
changing to doubleOptIn
Merge pull request #224 from segmentio/update/mailchimp

Adding flag for double opt in
Merge pull request #226 from segmentio/fix/google-analytics

Add support for GA screen resolution
Release 1.2.15
Attribution: Send Context