Add transform class for Codable types

[jvk75]

Enables Swift built in Codable types to be used with ObjectMapper.

Usage:

    required init(map: Map) throws {
        items   = try map.value("items", using: CodableTransform<CodableModel>())
    }

    func mapping(map: Map) {
        items   >>> (map["total"], CodableTransform<CodableModel>())
    }

[tristanhimmelman]

Would be happy to merge this, but can you please add a few simple tests so we can ensure its functionality moving forward?

[jvk75]

Sounds good, will do :)

[tristanhimmelman]

Dont worry about the broken test, I fixed that in the master branch. Thanks for adding in the testing