Fix TabsRoute behaviour

TabsRoute should implement two behaviours:
- it should save the state for each child route and restore it when switching tabs
- it should reset a child's state if that child is being navigated to _twice_

Given the following route config:

``` js

```

Visiting routes should lead to the following results respectively:
1. /foo --> show /foo
2. /foo/a --> show /foo/a
3. /foo/b --> show /foo/b
4. /bar --> show /bar
5. /foo --> **show /foo/b (a tab route saves and restores its children's state)**
6. /foo --> **should reset stack of the child stack route and show /foo**