Added intial selling/purhcase offer clarifications

Added some initial specification on how to deal with Selling & Purchase offers. We can discuss these changes here, I don't consider this PR final yet so it should probably not be merged yet.

Please see my [post here](https://bitcointalk.org/index.php?topic=292628.msg3621862#msg3621862) to see my opening statement about impartial transactions and how to deal with them.