Remove subsets from input trees

As in https://github.com/fb55/DomUtils/pull/4 , I wasn't sure where to place the tests for this functionality. Let me know if there's a better place for them!

This should resolve GitHub issue #12.

Commit message:

> When multiple DOM structures are specified via an array, it is possible
> some structures contain others as subtrees. Since selecting elements
> across these trees may lead to duplicate results, subsets ought to be
> removed prior to selection.