Renamed some collection methods.

Added IsEmpty normal property.
Make ScheduledSubject more flexible.
Added ListenIncludeLatest method to MessageBus to receive last sended message.
Merge pull request #218 from reactiveui/rxui5-misc-jlaanstra

Misc improvements from jlaanstra/reactiveui
fix one reference to `CollectionCountChanged` -> `CountChanged`
Merge pull request #221 from wendazhou/rxui5-master-collection-name

fix one reference to `CollectionCountChanged` -> `CountChanged`
Merge remote-tracking branch 'origin/rxui5-master' into portable
Define PORTABLE in Debug mode