Make Node an Enumerable

[gjtorikian]

Closes #23.

@dgraham, how's this work for you? walk has been renamed each so that all the Enumerable methods can function. I only added a test for select but can add more if you think there's something to explicitly test.

[dgraham]

The use case I was working with was iterating through a node's direct children without descending into the tree. So renaming each_child to each would fix that scenario.

Another possibility is to return an Enumerator from each_child if no block is given, similar to Array#each. That would allow this usage:

items = parent.each_child.select { |child| child.type == :list_item }

[gjtorikian]

Cool, I got you. I renamed each_child to each (well, it's an alias, because I didn't want to do a major version). Anything else you'd like to see?

[dgraham]

It might be nice to return an Enumerator from walk, if no block is given, to match the behavior of each.