Refactored All APIs for Swift 3

[cnoon]

The goal of this PR is to refactor ALL APIs used in the framework and test target source code to align with the Swift 3 API design guidelines.

The codebase has also been reorganized to make it easier to subclass the SessionManager, SessionDelegate and Request by eliminating excessive use of extensions by splitting core functionality up by feature.

The new top-level APIs now can be used as follows:

// Data Requests
Alamofire.request("https://httpbin.org/get", withMethod: .get, parameters: ["foo": "bar"])
Alamofire.request(urlRequest)

// Download Requests
Alamofire.download("https://httpbin.org/get", to: destination, withMethod: .get)
Alamofire.download(urlRequest, to: destination)
Alamofire.download(resourceWithin: resumeData, to: destination)

// Upload Requests
Alamofire.upload(imageURL, to: "https://httpbin.org/", withMethod: .post, headers: headers)
Alamofire.upload(data, to: urlString, withMethod: .post, headers: headers)
Alamofire.upload(imageStream, to: urlString, withMethod: .post)

Alamofire.upload(
    multipartFormData: { multipartFormData in
        multipartFormData.append(uploadData, withName: "upload_data")
        formData = multipartFormData
    },
    to: urlString,
    withMethod: .post,
    encodingCompletion: { result in
        switch result {
        case .success(let upload, _, _):
            upload.responseJSON { response in
                debugPrint(response)
            }
        case .failure:
            debugPrint(result)
        }
    }
)

There are also several refactored classes, structs and enums including:

• Method is now HTTPMethod in ParameterEncoding.swift
• Manager is now SessionManager in SessionManager.swift
• Manager.sharedInstance is now SessionManager.default in SessionManager.swift
• Manager.SessionDelegate is now SessionDelegate in SessionDelegate.swift
• Request.TaskDelegate is now TaskDelegate in TaskDelegate.swift
• Request.DataTaskDelegate is now `DataTaskDelegate in TaskDelegate.swift
• Request.DownloadTaskDelegate is now `DownloadTaskDelegate in TaskDelegate.swift
• Request.UploadTaskDelegate is now UploadTaskDelegate in TaskDelegate.swift

I've also updates all the public docstrings to use the new Swift 3 formatting syntax. I fixed several issues within the documentation and updated them to all match the new APIs.

The diff here is absolutely horrendous. I would suggest anyone looking to review this pull the branch and look at the changes directly in Xcode.

[jshier]

I think the order of method and urlString need to be switched here.

[cnoon]

Yep! Addressed in 2a6ccac.

[cnoon]

Thanks for the review @jshier. I'm going to go ahead and merge this then repoint the piggy backed PR to the swift3 branch.