fix(angular1_router): rename router component binding to $router

The current router is passed to the current component via a binding.
To indicate that this is an angular provided object, this commit
renames the binding to `$router`.

BREAKING CHANGE:

The recently added binding of the current router to the current component
has been renamed from `router` to `$router`.

So now the recommended set up for your bindings in your routed component
is:

```js
{
  ...
  bindings: {
    $router: '<'
  }
}
```

Author: petebacondarwin

modules/angular1_router/test/integration/router_spec.js

@@ -139,12 +139,8 @@ describe('router', function () {
       bindings: options.bindings,
       controller: getController(options),
     };
-    if (options.template) {
-      definition.template = options.template;
-    }
-    if (options.templateUrl) {
-      definition.templateUrl = options.templateUrl;
-    }
+    if (options.template) definition.template = options.template;
+    if (options.templateUrl) definition.templateUrl = options.templateUrl;
 
     applyStaticProperties(definition, options);
     $compileProvider.component(name, definition);