Correctly handle TimelineRangeOffsets (rangeStart / rangeEnd)

[bramus]

This PR updates how the polyfill handles offsets:

• Rename the delay/endDelay options to rangeStart/rangeEnd, as per spec.
• Remove timeRange option
• Remove animation-delay and animation-end-delay parsing from the CSS parser. Authors should use animation-range.
• Internally rename timeRange to animateRange and ANIMATION_DELAY_NAMES to ANIMATION_RANGE_NAMES
• Add support for px values as the range offset along the way

Fixes #64

[kevers-google]

Thanks Bramus.