Distinct 3 states for skipDelay: skipOffset not found -> skipDelay = def...

Distinct 3 states for skipDelay: 
- skipOffset attribute not found --> skipDelay = null
- skipOffset = -1 --> ad not skipable
- skipOffset = number --> skipDelay = skipOffset

We don't touch the creative.skipDelay property in the VASTTracker constructor, because we want to let the possibility to override the skipDelay manually via the VASTTracker.setSkipDelay().

By default, if no skipDelay is found and the skipDelay is not overridden, we act like the VAST spec (linear creative non skipable).