Check for selector, not class when retrieving request object from AFURLSessionManager

The class of the object associated with a notification sent from AFURLSessionManager may not be NSURLSessionTask e.g. [[notification object] class] => __NSCFLocalDataTask.
Check for respondsToSelector: in order to retrieve the request instead.
Fix podspec deployment target
Merge pull request #4 from jipeprojects/master

Fix podspec deployment target
Merge pull request #2 from arlophoenix/master

Check for selector, not class when retrieving request object from AFURLSessionManager
[Issue #1] Adding #ifdef around NSURLSession-specific declarations
Bumping version to 2.0.1
Limit the AFNetworking dependency to the NSURLSession subspec
Merge pull request #10 from phatblat/subspec

Limit the AFNetworking dependency to the NSURLSession subspec