Add ActiveRecord::Timestamp.skip_timestamps to skip timestamps easily

I've written following code many times.

```rb
Model.record_timestamps = false
@model.update(foo: bar)
Model.record_timestamps = true
```

It's very boring to write, so I added `skip_timestamps`. With the method, you can write:

```rb
Model.skip_timestamps do
  @model.update(foo: bar)
end
```