Fix {{#with view.foo as bar}}

The `with` helper can correctly bind through the `view` keyword when
it's used like this:

    {{#with view.foo}}

But the binding is broken if you try it like this:

    {{#with view.foo as bar}}

This change fixes it.
Expand Route#render options documentation

This also updates it to reflect the additional `model` param you can
pass (enabled since 33a9891bff06ccce9bbc66e2acce9360f13ecdbd)
[BUGFIX beta] Force remove `required` attribute for IE8

In IE8, `elem.prop('required', '')` couldn't remove attribute.
(This problem is occured at `required` only. Other attributes work as our
expectation.)

``` javascript
view.set('required', false);
view.$().get(0).outerHTML; // this includes `required` attribute.
```

To remove attribute, `removeProp` is needed.