Rename "generator expressions" to "generator comprehensions."

The spec uses this terminology (even though the outdated wiki page does not). "Generator expressions" are actually the generator counterpart of "function expressions."
Fix iterator test for latest ES6 semantics.

The previous test was just a `for`-`of` syntax test, looping over an object which is not iterable and doing nothing with the result. The new test makes use of the `Symbol.iterator` property to install an iterator on an object, making it iterable, and then tests that using `for`-`of` on that object iterates once, giving the correct value.
Merge pull request #83 from domenic/better-iterator

Rename "generator expressions" to "generator comprehensions"; fix iterator test.