Added the playsupported attribute

There’s some logic to determine if we’re on a platform that supports
programmatic playback, and reflect that in the attribute.
Changes to playsupported detection.
Cleaned up some comments.
Bump version to 0.0.9.
Cache playsupported in localStorage after detecting it.
Merge pull request #17 from GoogleWebComponents/playsupported

Added the playsupported attribute