First cut of generator samples.

@ebidel @addyosmani & co.:

Here's my initial code for a comprehensive example of iterating using generators.

I need to add some explanatory text and there's going to be a complementary H5R article, but I was hoping to get the code reviewed first.

Many of the existing samples related to generators gloss over the gnarly bits, so I deliberately contrived a more complicated use case to show off as many aspects as I could.

Live demo at https://rawgit.com/jeffposnick/samples/es6-generators/generators/index.html
This is not quite right. There is an explicit `return undefined` which yields a `{done: true, value: undefined}` and ends the generator.