HG-191 Re-implement Google Analytics event tracking

* Added new GA tracking module
* Changed structure of GA config in localSettings
* Changed UA implementation to older method using ga.js
* Moved tracking JS to head element
* Corrected some typos ;)
HG-191 Use interface for type
HG-191 Code review feedback
HG-191 Additions to GA tracking

* Addressed code review feedback
* Implemented singleton for event trackers
* Added new GA tracking util which wraps page view track
HG-191 Check special wiki for GA
HG-191 Add Mercury property and address comments
Merge pull request #248 from Wikia/HG-191

HG-191 Re-implement Google Analytics event tracking