Repeat action causes small stutter

When repeating an action (e.g. rotation) each time the action repeats their is a small stutter.
This is caused by the action reset because an action rarely ends at a completion ratio of exactly one.

Changing

```swift
internal func reset() {
startTime = nil
lastUpdateTime = nil
}
```

to

```swift
internal func reset() {
startTime = lastUpdateTime
}
```

in ```Action``` seems to fix the problem in the case when an action is repeated but could potentially cause problems in other cases. Maybe ```Action``` should have a ```restart```method? In that case the responsibility for calling ```reset```or ```restart``` probably needs to be moved out of ```ActionWrapper```.