Use regenerator 

Move the sample over to use the branch of [regenerator](https://github.com/facebook/regenerator/pull/101) which supports async/await.

There are two current limitations in the support provided by regenerator - I'll update the sample as these are fixed:
* Async arrows are not supported yet (and there are open questions about the syntax)
* There is a bug in handling of async function declarations preceeded by statements