Don't update NSProgress for data tasks

1. Progress is never returned by data task methods, only download methods. Those methods update the progress elsewhere so removing this shouldn't affect them.
2. The totalUnitCount is never being set during data tasks, so even if it was being returned, the NSProgress always reports a fractionCompleted of 0 right now.
3. There is a crash that occurs here rarely. My best guess is that the delegate is still being sent messages as it's being released.
[Issue #2046] Updating README about http session manager reachability
Merge branch 'master' of github.com:AFNetworking/AFNetworking