Mock GitHub doesn't allow control over timestamps

It does not currently seem possible to control the created_at (& other) timestamps of Mock GitHub entities. This makes it annoying or impossible to use the mocks for testing code that depends on the amount of time elapsed between mocked GitHub occurrences.
My suggestion would be for `Github.Time` to take a [`Clock`](https://docs.oracle.com/javase/8/docs/api/java/time/Clock.html)-like object as a constructor parameter, and use this `Clock` instead of `new Date()` to get the "current" time.