v1.0.0 add Action.follow(...), add .easeInOut() helpers

[reececomo]

What's changed?

• Implemented the four SKAction.follow(_:CGPath, ... ) actions as two methods:
  • Action.follow(path, duration), and
  • Action.followAtSpeed(path, speed)
  • Implementing reversing, orientation, etc.
• Added sensible global helpers for easing timing modes:
  • Chainable helpers for .easeInOut(), .easeIn() and .easeOut() helpers to access.
  • Global constants for default easeInOut, easeIn and easeOut.
  • Defaults to Sine family of curves, can be set easily.
• Docs improvements
• Renamed scaleXBy and related to scaleByX for consistency with other methods.