Create an Observable that emits each char in the source String.

Motivation:
* RxJS has this [from](http://xgrommx.github.io/rx-book/content/core_objects/observable/observable_methods/from.html) feature and it's simple awesome, so, why not included it :)

Example
```java
Observable.from("foo").subscribe(new StringPrinterObserver());
```

Output:
```
onNext : f
onNext : o
onNext : o
onCompleted
```