Support generator.return(argument) method for closing generators.

Closes #107.
Add a basic test of generator.return(argument);
Make sure generator.return(argument) works for newborn generators.
Merge pull request #109 from facebook/return-method

Implement generator.return(argument) method for closing generators.

Now officially part of the ES6 spec: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator.prototype.return