1.2.0 release

New in this release:

• Error-reporting mechanism.
• Variants of Subscription.multi(...) that "subscribe" to arbitrary elements.
• Subscription.dynamic(ObservableSet<T>, Function<T,Subscription>) to dynamically subscribe to elements of an observable set.
• EventStreams.sizeOf(...) factory methods to emit the current size of an observable collection.
• Added Connectable interface for objects that are capable of lazily subscribing to event streams.
  • Added ConnectableEventSource, an event source that lazily subscribes to any number of event streams, i.e. it is subscribed to its input streams only when it itself has at least one subscriber.