Transaction fix

I think I found the answer to issue #51. In connection.py, line 419 (master) the transaction() method yields back to the io_loop before beginning the transaction on the underlying connection. It seems like it's possible for another request to get the Connection at this point, because busy() will still return False.

I am using a bool in_transaction_method to guard against this.

Ignore my change to the version number in commit d967fc1.