Adds Checks for Successful Trade Completion.

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).
Merge pull request #322 from cwhelchel/fix/tradeclose

Fix for successful trade completion.