Add support for microseconds to `Duration`.

- Add `Duration::microseconds()`
- Add `Duration::totalMicroseconds()`
- Add `Duration::fromMicroseconds()` factory method
- Add `Duration::resolveToMicroseconds()`
- Add microseconds parameter to `Duration::fromComponents()` factory method 
- Change `Duration` to be based on microseconds but keep `$seconds` constructor argument to prevent BC break, also allow a `float` to be passed.

See #77
