Support for multiple GA trackers

Resolves https://github.com/luisfarzati/angulartics/issues/129

Possibly a more concise/elegant way & I wasn't quite sure how/where best to comment but basically...
 * When using the GA Universal analytics.js, if one or more trackers are created on a page (even if one is unnamed), anytime a tracking request is sent, I'm getting the internal name of each tracker and sending the request for each.  I've iterated over `ga.getAll()` each time, but open to suggestions if there's a smarter way. 

See GA docs: https://developers.google.com/analytics/devguides/collection/analyticsjs/advanced#multipletrackers for more info

Oh, a note on https://github.com/RideAmigosCorp/angulartics/commit/e9d5cab1a91c914734500a59a652328cf49b1bbb#commitcomment-5973389 Not sure if that needs attention or not. 

Eek.  This might be my first ever *real* PR on a **real** project.  I'm nervous :bomb: