Memory leak fixed. ChangePassword controll added.

Major memory leak fixed through implementing IDisposable and clearing command bindings, unsubscribing from events, etc. Lots of minor adjustments done to elements, potentially causing memory leaks.
Bug fixed in UserRepository (Now properly uses DbNull.Value instead of String.Empty as OleDbParameter)
Now every user can change his/her own password through a new user control - ChangePasswordControl.