Make Timeline.currentTime a read-only property rather than a method

See https://github.com/web-animations/web-animations-js/issues/107
Use Object.defineProperty() rather than deprecated __setProperty__() and getProperty__()

This fixes https://github.com/web-animations/web-animations-js/issues/106
Merge pull request #110 from steveblock/removeGettersAndSetters

Remove getters and setters