Use `NSURLSession` instead of `AFNetworking`

[mikeger]

Issue Summary

AFNetworking is a major, well-known library, however not every project is willing to integrate it. There could be several reasons, some of them could be:

1. Project is Swift-based, so AlamoFire is already integrated in the project
2. Project is based on NSURLSession, so the developer could find annoying to include the huge library to support downloading of preview images for videos

I realise that previously NSURLRequest based API was not the best, but now the NSURLSessison is as good as third party APIs.

Would be great to provide the version of VIMNetworking that does not have any network library dependency and works directly on Apple API.