Batch Action forms

FYI: after I get some feedback, I'll [convert this Issue into a Pull Request](http://opensoul.org/blog/archives/2012/11/09/convert-a-github-issue-into-a-pull-request/)

I want to add optional forms to Batch Actions; an example use-case being that you want the user to add in a text box the reason why they're taking this action. Since we're already including jQuery UI and it has [a similar UI for pop-over forms](http://jqueryui.com/dialog/#modal-form), a bit of the work has already been done.

What I'm curious about is what you think a good DSL syntax would be for this--should I include a `form do |f|` block **inside** a `batch_action` declaration, or should they be defined much like the new/edit pages are, with something like the below syntax?
```ruby
batch_form :for => :an_action do |f|
  #...
end
```