When compiling for .NET 4.5 or newer, use ExceptionDispatchInfo to preserve the original stack trace of an exception that occurs when forcing evaluation of a list-cell's value, so that if the cell's value is retrieved again, the exception can be re-raised with the preserved original stack trace.
Renamed the 'ObservableUpdate<T>' type -- it is basically the same as the 'Notification<T>' type from Rx.NET, so we'll re-use the same terminology to reduce confusion.
Merge branch 'master' of github.com:jack-pappas/ExtCore