Updates progress callback to use NSInteger

Pull Request #606 resolves this issue in the code but not in the Documentation.
Merge pull request #685 from jeffleeismyhero/patch-1

Updates progress callback to use NSInteger
Merge branch 'master' of github.com:rs/SDWebImage

# By Jeffrey Lee
# Via Jeffrey Lee (1) and Olivier Poitrey (1)
* 'master' of github.com:rs/SDWebImage:
  Updates progress callback to use NSInteger