Be more consistent about types

For more details see https://github.com/videojs/videojs-contrib-hls/issues/103
@dmlap Sorry for the back and forth on this one. I was running the wrong unit tests. When I did this like was triggering 12 errors. The version that is now checked in uses fillbuffer without function wrapper but does *not* normalize fillbuffer's offset parameter. 
If I had to guess I'd say that there is code out there that relies on having offset stored as an Event object in segmentBuffer. I propose revisiting this problem later and move on with the rest of this PR. Agreed?