AFNetworking 2.1 support

AFNetworking is already on 2.1, so I’ve decided to make a straightforward port.

If you want to use this code with AFNetworking 2.0, you’ll get method deprecation warnings. Use the `2.0` branch to prevent them:

```
pod 'AFAmazonS3Client', :git => 'https://github.com/evoL/AFAmazonS3Client.git', :branch => '2.0'
```