Plain types parsing

[maxkonovalov]

This makes possible to map plain swift types like this:

func getArticlesCount() -> Promise<Int> {
    return ws.get("/articles", keypath: "count")
}

Need to figure out a better way to parse RawRepresentable types to pass all the tests.

[maxkonovalov]

Also added some minor changes to WSError:

• changed Type enum to Status to prevent collisions with swift built-in Type
• added "Unable to parse response" error

[s4cha]

Another Great feature to write more concise code 🎉👏