Add store_location_for helper

This is used as a complement to `stored_location_for`.

Example:

Before authorizing with Omniauth;

``` ruby
store_location_for(:user, dashboard_path)
redirect_to user_omniauth_authorize_path(:facebook)
```

In our Omniauth callback

``` ruby
sign_in(user)
redirect_to stored_location_for(:user) || root_path
```