ST3 support

In case you are interested, I've updated SmartMarkdown to work with SublimeText 3 beta (at least to some extent). However, I'm not sure if those changes could be merged in some way, If you are interested in those at all. The problem is that those fixes are incompatible with ST2, so I've created a separate `python3` branch in my fork, but have no clue how to push those properly. There's also that UNOwen/SmartMarkdown@9a8f01f20f7f1afe87e58b87d63e9463f6283673  (handle headline_move error) commit, which is unrelated to ST3, and should probably be considered separately. I mean, I've probably screwed up and made the process more complicated than it should be, but what can you do...