Add support for skippable Linear creative/progress event with offset/tests

Proposed changes to resolve #6 - skippable linear ads

To make a linear ad skippable, extra parameter: 
`skipoffset: '00:00:05'`
needs to be provided to attachCreative.

In addition, to comply with the VAST3.0 spec, one can attach a progress event with the required offset via:
`.attachTrackingEvent('progress', 'http://zing-zang.com', '00:00:30.000')`
third optional argument.

Feedback very welcome, thanks!