WikiPage.revision_by now grabs proper username (issue #258)

This hack seems to resolve the issue, and as far as I can tell doesn't break anything. You guys know the codebase better than I do at this point, so let me know if you think this is a good solution. Some manual tests:

    >>> r = praw.Reddit('sdf', site_name='local')
    >>> w = r.get_wiki_page('reddit_test0', 'wiki1')
    >>> w.revision_by
    Redditor(user_name='reddit')
...

And on the /r/webdev wiki index, where we'd expect the latest revision to have been made by user 'jaredcheeda':

    >>> import praw
    >>> r = praw.Reddit('PRAW-dev testing wiki revision grab')
    >>> w = r.get_wiki_page('webdev', 'index')
    >>> w.revision_by
    Redditor(user_name='jaredcheeda')

For unit testing there'd have to be a static wiki page for which we always know which user we expect to have revised. Or we could run it against a private reddit.com sub where no changes are planned (I think I remember seeing @bboe using this method in the code already).