Add kf namespacing

[onevcat]

This p-r adopts to a new namespacing system. It could avoid possible naming conflicting by introducing an extra struct layer. So we could get rid of the kf_ prefix. Instead, we could use an easier kf. to limit Kingfisher related APIs in its own namespace.

[webfrogs]

Looks great. But I have some different ideas about the implement of namespace.It's about adding namespace extension with Struct type in swift.

1. Using the generic struct Kingfisher<Base> as the default implement for Class Type
2. When extension the Struct type like Swift 3's Data type. Define a new struct named like XXXProxy to implement the KingfisherCompatible type which is defined before. Then writing all extension of the Proxy struct to do the extension jobs. By doing this, We can avoid using type cast to get the right type, because I believe type casting is the bad code smell of swift.

First, making some small change of KingfisherCompatible protocol, There is no need to give the set property.

protocol KingfisherCompatible {
    associatedtype CompatibleType
    var kf: CompatibleType { get }
}

Then, extension Data type can be something like this:

struct DataProxy {
    fileprivate let value: String
    init(_ value: String) {
        self.value = value
    }
}

extension Data: KingfisherCompatible {
    typealias CompatibleType = DataProxy
    var kf: CompatibleType {
        return DataProxy(self)
    }
}

extension DataProxy {
   // Do whatever you want, and you can use value to get the exactly type: Data
}

[webfrogs]

#441 , I create a pr ,just change the way of implemention