Standardized ga.js and analytics.js check calls

* There was a lot of if (ga.js) { ... } else if (analytics.js) { }
statements throughout the code, but they weren't always consistent in
how they were defined. Replaced this with a callback style function call
to check and then execute if the appropriate script is loaded.
* Added a few more tests for trackEvent to test the non-interaction
parameter
* Improved trackPage and trackEvent tests to validate the ga construct
states.