Add initial mvt ab cookie.
Allocate users to ab test using mvt cookie value

Move ab test initialise out of app, into ab module
Allocate users into fixed value ranges.

Generate mvt cookie before omniture tracking.
Fix ab tests
Merge branch 'master' of github.com:guardian/frontend into unique-id-cookie

Conflicts:
	common/app/assets/javascripts/bootstraps/app.js
Remove line break
Merge pull request #2469 from guardian/unique-id-cookie

Unique id cookie