Adding google tracking code for ehcache.org

[akomakom]

No description provided.

[mathieucarbou]

@ljacomet : I wonder if the PR is complete because analytics should be activated only on ehcache.org, and not locally when testing the website. For that there are several options:

1. JS code to conditionnaly init the tracker only if the document.location contains ehcache.org
2. Templating code to test if the generation is done locally or on server (env var perhaps?)
3. I know that in google analytics, we can filter out some origins (that is what I already did since all the dev team was accessing from the same static IP.). I don't know if we can filter out based on the URL (i.e. exclude localhost:4000/*)

[akomakom]

If the local testing is done via jekyll, we can have a boolean variable that's only set for production. (Meaning a variable passed to jekyll on the command line)
Specifically JEKYLL_ENV :)
I'll make a PR for that