node relevant at the root, but not in src. Broser relevant in src, but not in root.

null !== undefined

Pin React devDependencies, add peerDependencies

When the package is installed as a dependency, it should warn the installer that React is a peerDependency.  When installed this way, the devDependencies won't be installed, so only one version of React will be present. However when it's installed as a standalone project, React should be installed automatically as a devDependency.

I think this gives us the best of both worlds — simple development setup as well as supporting usage as an installed package.

Initial rough sprite/costume/backdrop libraries

# Conflicts:
#	package.json
#	src/containers/gui.js

Makes local deployment easier

So that you can adjust them without redefining the entire GUI.
Also allow adjusting props passed to GUIComponent.

So that it's easier to use these standalone.

`npm update` does what we want and is less hacky than removing the packages entirely first.  Specify latest as the version so we always get the latest version even though I erroneously installed scratch-blocks@0.1.0.

And use instanceOf for the vm prop type

Move behavior-related components to "containers", leave presentation components in "components".

Confine the logic for adding/removing event listeners between the VM and the components to the relevant components.  While this tightens the coupling between the component and the environment, it makes using the individual components more self-contained.