Fix errors when called without an AbstractPage object

This apparently happened when trying to access non-existant images in one of our projects.
Add twig function to initialize Google Analytics script.

Untested for now.
Split out Google Analytics helpers + add ecommerce tracking helper
Provide documentation and tweak twig functionnames
Cleanup google Analytics init script

Only log when console is defined, don't input the exports on the selfinvoking function, bind the click immediately and finally also log actual button clicks.
Pull out conversion logic + deduplicate the orderitems before rendering the template
Fix CodingStandards
Merge pull request #18 from Kunstmaan/feature_generic_ga_script

Add Google Analytics Script