Define expectNoElement in terms of expectElement and add tests

You have this line in the README:

* ensure that expectNoElement(selector, {contains:text}) works

I found today that it doesn’t.

This change defines `expectNoElement(selector, options)` as an actual convenience method for `expectElement(selector, 0, options)`, adds tests to ensure functionality, and removes that line from the README.

Maybe you defined it differently because you wanted `expectNoElement`-specific failure messages, but it seemed sensible to me to just reuse the existing code rather than define a helper with most of the same functionality.