Fix for successful trade completion.

This should partially resolve some issues whereby trades weren't closed out properly and the timeout thread still ran. This caused some people to either restart their bots or wait for the timeout to occur. 

The code here's pretty easy. A new `trade_status` value needed to be checked in the `Poll` and exposed properly.

This doesn't necessarily fix issues with trades that went tits-up and failed. 

>Commit log:
>This checks the trade status for trades that end successfully. It then shuts down
the trade normally. The Trade.OnClose event is fired correctly as well. This fixes
long-standing bugs where a bot couldn't accept trades until it timed out (as if
it was still in trade).