Capture slot context for timer and download

Fixes #876 and addresses the `download` problem noted at https://github.com/shoes/shoes4/issues/617#issuecomment-64933275

Both `timer` and `download` take action asynchronously, but can be called from within a nested slot context. For instance, take this test app:

```
Shoes.app do
  stack do
    background blue
    para "Nothing"

    timer 1 do
      para "Something"
    end

    download "http://jasonrclark.com" do |dl|
      body = dl.respond_to?(:read) ? dl.read : dl.response.body
      para body
    end
  end
end
```

Although the `timer` and `download` make it look like their UI will show up within the blue `stack`, we instead get this result:

![image](https://cloud.githubusercontent.com/assets/130504/5258529/ebb70120-79a4-11e4-96ee-4955631ae3a5.png)


This PR captures the context of the `current_slot` with the `timer` and `download` blocks, and wraps them setting that slot context again when they are run. Now we get everything inside the `stack` like you'd expect:

![image](https://cloud.githubusercontent.com/assets/130504/5258538/0c12848a-79a5-11e4-8931-e887ed3436ac.png)


The only other case that I've found with this type of delayed behavior is `animate`, but I'm very leery of changing that since we've established elsewhere that animations don't seem to act in any way like they live within the slot they're called from.