Remove "root" node

This should resolve issue #162. Note that I had to change some tests, but this was only necessary in those cases with explicit references to the `root` object.

Commit message:

> Cheerio uses a "root" object internally to collect parsed markup with
> multiple top-level nodes. This object is not itself a node, but
> modelling it as such can interfere with the parsing and traversing logic
> that Cheerio relies on.
> 
> Attach the "root" object via an API that is not recognized by the
> traversal logic so that it is not exposed through querying operations.