Refactor + basic Android support

[elliottsj]

Summary:

• Replace --entry option with --androidEntry and --iosEntry.
• Replace --bundlePath option with --androidBundlePath and --iosBundlePath.
• By default both platforms are built, unless --no-android or --no-ios are passed.
• Remove default values in Server constructor: default values now only exist in rnws.js.
• Start RN packager and webpack dev server upon calling .start() instead of upon new Server().
• Properly handle requests for both /index.ios.bundle and /index.android.bundle
• Use arrow functions (requires Node.js v4)

Resolves #65

e.g.

(requestAnimationFrame doesn't seem to work in Android yet, but at least something renders!)