replace children sugar

Jamon and I discussed this new feature, you can do this now with rmq, but this is symantically very nice:

``` ruby
rmq(@my_view).replace_children do |view_q|
@emails.each |email|
view_q.append(UILabel, :email_address).data(email.url)
end

view_q.size_to_fit
end
```

`replace_children` will simply remove all children, then return `rmq(@my_view)`, which can be used with Jamon's new **do** syntax. So really replace children is just a nicer way to do `rmq(@my_view).children.remove.back` (assuming that works with remove).

What do you fellow contributors think?

Start from the rmq(@my_view) entry point and inspect the existing children.remove.back behavior mentioned in the issue. Implement the proposed replace_children behavior around that existing chain, preserving the block/do syntax, and verify that children are removed and the view queue remains usable for appending new views.

Written by the indexing model from the issue text.