New operation on RACStream to filter out nil objects

*(Alert: noob PR) I just recently started learning ReactiveCocoa, so it's possible that this PR either re-implements something that already exists or is simply terribly wrong :) In either case I will learn something, so I can't go wrong.*

In every RAC article that I read, I saw examples doing:

```objc
[someStream filter:^ BOOL (id value) {
    return (value != nil);
}];
```

And it makes sense, filtering out nil values must be a pretty common thing to do, so I think having a simpler method that does exactly this could simplify a bunch of code.

I'm not 100% convinced the name I chose (`-filterOutNil`) is the best one, I accept critics :).