Add ReactiveList<T> derived class that supports IObservableCollection<T>

[fahadash]

Caliburn.Micro has IObservableCollection which represents what we might call ReactiveList in ReactiveUI. But currently ReactiveList is not readily assignable as IObservableCollection. We might need to add a Type so we can do this

IObservableCollection myNumbers = new ReactiveCaliburnList();

[Damien Chaib]

It has been suggested in #20 as well. Do you have any other use case in mind?

[Damien Chaib]

Done with #23.