Add notion of top level `unit` that creates stream

of application changes itself.

unit.js

@@ -1,5 +1,7 @@
 "use strict";
 
+var channel = require("reducers/channel")
+var pipe = require("reducers/pipe")
 var filter = require("reducers/filter")
 var map = require("reducers/map")
 var flatten = require("reducers/flatten")
@@ -27,13 +29,20 @@ function unit(mapping) {
         count: component(null, itemCountWriter)
       })
   **/
-  return function reactor(changes) {
+  return function reactor(source) {
     /**
     Function takes stream of state changes for the unit it's responsible
     and distributes change across nested components / units. As a return value
     it returns joined steam of changes caused by interactions on all nested
     components in same structure as stream of changes it reads from.
+    If `
     **/
+
+    // If `source` stream of state changes is not provided this means this
+    // is reactor for top level unit. So all the change events generated by
+    // it's entities will be all the application state changes, there for
+    // channel for that changes is created.
+    var changes = source || channel()
     var inputs = keys(mapping).map(function forkChanges(id) {
       var reactor = mapping[id]
       // Filter stream of changes to a stream of changes to the entity
@@ -49,8 +58,10 @@ function unit(mapping) {
     })
 
     // Return joined stream of all inputs from all the entities of this
-    // unit.
-    return flatten(inputs)
+    // unit. Also if it's top level unit (`source` was not passed) pipe
+    // state changes back to the application state changes as all the changes
+    // are caused by it's entities.
+    return source ? flatten(inputs) : pipe(flatten(inputs), changes), changes
   }
 }