Implement `removeSubsets` method

Hi @fb55,

This functionality is necessary to resolve https://github.com/fb55/CSSselect/issues/12 (and to simplify the solution for https://github.com/MatthewMueller/cheerio/pull/289). It seems odd for it to live in `CSSselect`, so I'm submitting it here.

I'm sure you'll have a lot to say about the code organization, but I think this project could use a little more structure to its source files. I just threw the tests in an arbitrary location, so please let me know how they should be structured, too.

Commit message:

> This allows other libraries to ensure that manually-created collections
> of nodes contain only unique trees.