Add new constructors to create EventStreams from Iterables and Futures
Remove unnecessary dynamic extension

Looks like they fixed the bug in Dart
Add Reactable.collect that returns a Property of the latest reactable values

This is a static method on `Reactable` that takes a collection of `EventStream`s
and `Property`s, and returns a `Property` where its value is the latest values
from the reactables.
Bump version to 0.3.1