fix($route): make nextRoute.$route private

the `nextRoute` object available in `$routeChangeStart` handler
accidentaly leaked  property which pointed to the route definition
currently being matched.

this was done just for the internal needs of the `$route` implementation
and was never documented as public api.

Some confusion arouse around why the $route property was not always
available on the `nextRoute` object (see #1907). The right thing for us
to do is to prefix the property with $$ for now and refactor the code
to remove the property completely in the future. Application developers
should use the `nextRoute` object itself rather than its `$route` property.
The main diff is that nextRoute inherits from the object referenced by $route.

BREAKING CHANGE: in $routeChangeStart event, nextRoute.$route property is gone.

Use the nextRoute object instead of nextRoute.$route.

Closes #1907

src/ng/route.js

@@ -397,7 +397,7 @@ function $RouteProvider(){
       var next = parseRoute(),
           last = $route.current;
 
-      if (next && last && next.$route === last.$route
+      if (next && last && next.$$route === last.$$route
           && equals(next.pathParams, last.pathParams) && !next.reloadOnSearch && !forceReload) {
         last.params = next.params;
         copy(last.params, $routeParams);
@@ -477,7 +477,7 @@ function $RouteProvider(){
           match = inherit(route, {
             params: extend({}, $location.search(), params),
             pathParams: params});
-          match.$route = route;
+          match.$$route = route;
         }
       });
       // No route matched; fallback to "otherwise" route