Don't register fillBuffer checks until after a media playlist has downloaded
Fixes #110. Fixes #116. If timeupdates were being firing without a media playlist available yet, fillBuffer would dereference a property on undefined and blow up. Timeupdates occur manually on a fixed interval when using the Flash tech so this was guaranteed to happen whenever the media playlist was unretrievable. To avoid this, wait to register buffer checks until a media playlist has been successfully downloaded. As it turns out, this is the same moment we normally trigger loadedmetadata so it was possible to transform a loadedmetadata handler into a direct call to fillBuffer.