Add wait command

When testing AJAXy/single-page apps, it is often required to simply wait a specified amount of time when something happens. While it is probably possible to do it using `waitFor()` with an element that does not exist, it is not intuitively clear and feels hacky.

This PR adds a simple `wait` command that simply uses `setTimeout` to wait for the given amount in milliseconds without trying to look up any elements.