Move immutable/* re-exports to immutable/index.js

[kale-stew]

All immutable imports now must be separately imported. For example:

import { immutableRouterForBrowser, ImmutableLink } from 'redux-little-router/es/immutable';
import { combineReducers } from 'redux-immutable';

Checklist:

• Move immutable/* re-exports to immutable/index.js
• Scour and update docs
• Scour and update examples

[codecov]

Codecov Report

Merging #266 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          28       28           
  Lines         362      362           
=======================================
  Hits          357      357           
  Misses          5        5

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[ryan-roemer]

LGTM! Let's get @tptee to review and double check there aren't any gotcha's I'm missing...

[ryan-roemer]

@stefvhuynh -- Mind taking a look at this too since we're moving around your immutable work?

[tptee]