Next/prev

Add next & prev fields to GumboNodes to traverse the tree in the order that nodes were added.  Also add them to the ctypes & BeautifulSoup bindings.

This was necessary to bring the BeautifulSoup adapter into compliance; turns out some of the lesser-used functions in the BS3 API rely on those fields having useful values.  It may be useful for other tools or language bindings as well.

As an API change, this should probably get a version number bump (0.10?  It's a little weird, not having reached 1.0 yet).  As a pure addition it shouldn't create any API compatibility problems, although it changes the ABI.  Comments?