Observable+FilterNil.swift

protocol OptionalType {
    associatedtype Wrapped
    func map<U>(@noescape f: (Wrapped) throws -> U) rethrows -> U?
}

extension Optional: OptionalType {
    
}

extension ObservableType where E: OptionalType {
    func filterNil() -> Observable<E.Wrapped> {
        return self.flatMap { $0.map { Observable.just($0) } ?? Observable.empty() }
    }
}