Chaining of Observables ("key path observing")

Added ability to chain observables (or "observe key paths"), e.g.

given class `Person` with `.friend` of `Observable<Person?>` and `.lastName` of `Observable<String>`

observe the lastName of given persons friend - wether the friend changes or the name.

This can now be achieve with fluid syntax:
```swift
chain(me.friend).to{$0?.lastName}.afterChange += { /* ... */ }
```
Or using the `/` operator
```swift
(me.friend / {$0?.lastName}).afterChange += { /* ... */ }
```