Add a drift() method that will add a given number of milliseconds to the next event call

The idea is to allow some simple syncing between different clocks. If we know the time drift between two clocks in milliseconds, then we can use `timer.drift()` to add or subtract that number from when the next event will run.

I've added a test demonstrating this. You can run it with `mocha --timeout 5000 test`. I also added documentation of the `drift()` method to the readme.