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
Upgrade recast to v0.7.1 and depend on esprima-fb instead of esprima.
Bump NPM version to 0.5.0 in preparation for publishing.

The minor version bump is due to the switch from esprima (harmony) to
esprima-fb. I don't anticipate any backwards compatibility problems, but
this change is potentially user-visible, so it needs to be optional.
Fix test broken in browsers where Symbol.iterator is actually defined.

Chrome dev channel now actually supports Symbol.iterator! Yay!
Bump NPM version to 0.5.1 in preparation for publishing.
Allow regenerator to be used as a Browserify transform.

Fixes #20.
Bump NPM version to 0.5.2 in preparation for publishing.
Replace wrapGenerator with regeneratorRuntime.wrap.

The .wrap method is now just like all the other runtime methods.
Bump NPM version to 0.5.3 in preparation for publishing.
Rename runtime/dev.js to runtime.js and get rid of runtime/min.js.
Revamp public API.

The public interface of Regenerator now more closely follows community
transpilation conventions:

  - require("regenerator") is a function that returns a writable
    stream for use with browserify -t (#20).

  - require("regenerator").compile is a function that transforms a string
    of source code and returns the result of recast.print.

  - require("regenerator").runtime() is now the preferred way to include
    the runtime.

  - The require("regenerator").runtime.{dev,min} properties are no longer
    exposed to client code.

I've been wanting to make these changes for a long time.
Bump NPM version to 0.6.0 in preparation for publishing.
Add runtime support for async functions.
Transform async functions and await expressions.

Closes #100.
Implement async function test suite, with Promise runtime polyfill.