Batch Action forms

FYI: this is an Issue that was [converted 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
```

#### UPDATES

* The syntax is now:
```ruby
    batch_action :foo, confirm: 'You sure?', form: {bar: :text} do |ids, inputs|
      inputs[:bar] # access user input from this hash
      # do your stuff!
    end
```

* I changed the behaviour of the `:confirm` option. Passing a string still works, but now if you pass `true`, it will use a default confirm message.

* The UI now looks like this:
![Modal styling](http://f.cl.ly/items/0P3U1V2d3v1Z20153V3L/Screen%20Shot%202013-05-01%20at%2011.11.35.png)