Export: Select methods to be exported to the window scope

Fixes #561

This patch doesn't try to solve or close the deprecation of logging callback export in https://github.com/jquery/qunit/blob/master/src/core.js#L528-L558

It only select specific methods to export to the window (global) object regarding retro-compatibility.

In another moment, we should deprecate these methods to a further use of namespaced methods, like `QUnit.*`, but that might be planned as even QUnit tests require them. Examples: test, module, expect, etc.