Add trackingEvents to Companion constructor; support multiple ClickTracking URLs (v2)

(Continued from pull request #38)

* Add empty `trackingEvents` to `VASTCreativeCompanion` constructor to keep it from exploding when `<CompanionAds>` exists but is empty
* Support multiple `<ClickTracking>` URLs (changes API)
* Add new tests
* Clean up some test description grammar

Sorry I took so long to get back to this.