Fix implementation of $.fn.children

According to [the jQuery documentation on
`$.fn.children`](http://api.jquery.com/children/):

> **Description:** Get the children of each element in the set of
> matched elements, optionally filtered by a selector.

Instead of only returning the children of the first matched element,
return the children of *every* matched element.
Re-factor $.fn.children

As per @davidchambers's feedback:

- use `_.reduce` rather than a for loop
- pass `isTag` to `_.filter` directly (the function wrapper serves no
  purpose)
- rename the collection `elems` to avoid shadowing `children` in outer
  scope
Merge pull request #150 from jugglinmike/148-fix-children

Fix implementation of $.fn.children