get_string_urls before putting urls into a set to avoid unhashable 'dict'

error when dealing with h-cite style in-reply-tos.

This ended up being almost trivial, alltests.py looks good. publishing RT's to twitter ended up being the more interesting test case (e.g., http://kylewm.com/share/2014/04/09/1) as Facebook doesn't support verb='share' posts.