Add .invoke() method to collections

[ricardobeat]

Since we don't have a few helper methods like focus(), would be nice to have invoke operate on a collection:

form.find('input').invoke('focus')

It could automatically map() over the elements for return values, unlike attr and other methods that return only the first value:

list.find('li').invoke('getAttribute', 'id') // => ['a', 'b', 'c']

[jcemer]

Sounds nice!