Add empty data field on auto-generated beacons

The server expects every beacon to contain a 'data' field.
Pass yogurt_user_id through the 'setYogurtUser' command
Read 'yogurt_user_id' on the ActionsManager
Use 'yogurt_user_id' on Session's create phase
Change default data value for auto-generated PageViews to string
Fix typo in sendPageView action name
Factor out session extraction code to session_engines
Add timeout for XDomain session extraction
Refactor Session