Fix some minor bugs in the link neighbour code

The traverser will now use Node:firstChild and Node:nextSibling to
traverse, and our link neighbour handler will now bail out correctly
if it's handling a non-wikilink link.

Test case:

echo "Writing 40 [[Greasemonkey]] scripts[http://userscripts.org/users/21031/scripts] for fix websites and support Firefox" | node parse --apiURL "https://en.wikipedia.org/w/api.php" --wt2wt

Also:

node roundtrip-test.js 'August Penguin'

Change-Id: I736b794a2b916350c148f419b5153190fcaecbcf
Merge "Fix some minor bugs in the link neighbour code"