Add support for last-child, first-of-type, last-of-type pseudo-selectors

[chrishow]

Adds support for pseudo-selectors :

:last-child
:first-of-type
:last-of-type

See #39

[g105b]

This looks great! It's functionality I've ignored implementing for so long. I'll check this out tonight and do the usual Dom integration test.

[g105b]

This is super. I've wanted to have this functionality for so long and keep on doing hacky workarounds like $document->querySelectorAll("li")[2] instead of :nth-child(3).

Thank you!