Refactoring: introduced GetUserId.
Refactoring: added FindUserById and UpdateUser utility methods.
Refactoring, TradeManager: introduced the "SendMessage" method to deal with notifications (so that we can add easily email notification later on).
Minor: added assertions for private methods.
Bugfix, exchange management, dates are wrongly interpreted when the server is run outside "en" cultures.
Exchange management: time is saved in GMT instead of local time.