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
Development workflow updates on Gruntfile

Enable Livereload on payload files.
Don't load minified payload on development.
Factor out session extraction code to session_engines
Add timeout for XDomain session extraction
Refactor Session