Add support to pass an object as initAction, specifying a part
that should run prepared and a part that should only be executed
on the client.

Update the existing spec to make sure unit test still pass

Create a IsomorphicTestEnvironment utility that can be used in
integration tests to simulate a component rendering on the
server first and then on the client. This can be used to
test more complex situations.

Create 2 sample integration tests using the new utility. This
file is named .test.js to distinguish from the unit tests, which
are named .spec.js

Create an ExtendableError utility to create custom error classes.
Use this to create a custom error type PrepareValidationError. This
can be used to assert that a prepare validation error has occured
in tests (as opposed to another type of error).

Add the babel-plugin-transform-builtin-extend plugin to make sure
we can extend the builtin Error correctly.

Allow to run the integration tests separately from the unit
tests for faster test development

Fix nested it testing blocks. They are not valid jest

Because initComponent is now called from both willMount and
didMount, we need to make sure that the initAction is only called
from one of these hooks

Add an update function to IsomorphicTestEnvironment. This will make
sure allow us to re-render the test renderer with new components.

Use the fixture component instead of rewriting a component every
time

Check that component has been prepared should not be done when
initAction === null (clientOnly init action)

Instead of writing the whole JSON for actions dispatched, use
snapshot testing. This improves readability in the unit test
files

All store test state is static, so we can move it to a fixture
file to save some space in the unit test files

Log a warning on redundant calls to prepareComponent() when the
component only has client-side initialization

Changed the server-side render in IsomorphicTestEnvironment to
react-dom/server renderToStaticMarkup(). This is to prevent
didMount hooks from being called.

Create integration test that includes both prepared and clientOnly
actions

Fixed wording in unit test description.

Included the clientOnly change to withInitAction docs

Updated formatting and styling of the docs

Temporarily changed the theme to a forked version to fix a charset
issue