Implemented NTLM auth support. Replaced deprecated auth challenge methods from `NSURLConnectionDelegate`. Replaces #711.
- added `username` and `password` properties on `SDWebImageDownloader`
- added `shouldUseCredentialStorage` and `credential` properties on `SDWebImageDownloaderOperation`
- `SDWebImageDownloaderOperation` conforms to `NSURLConnectionDataDelegate` (it used to implement the methods, but didn't stated the protocol in the declaration)
- removed deprecated methods: `- connection:canAuthenticateAgainstProtectionSpace:` and `- connection:didReceiveAuthenticationChallenge:` with `- connectionShouldUseCredentialStorage:` and `- connection:willSendRequestForAuthenticationChallenge:`
- updated demo project to download one image the requires HTTP auth