adds dom/children

As the doc already specifies. This method returns an array of child nodes excluding any node of kind `TEXT_NODE` or `COMMENT_NODE`. This makes it easier to loop over children of hand written html (containing line breaks etc.)

I wasn't to sure about two things though:
* The name `children()` might insinuate that it returns all children including texts and comments
* When an array of nodes is specified as `parent` it will return an array of all the children of every node. Is that the expected behavior?

What do you think @conradz