Update UIButton+AFNetworking.m

Added call to sharedImageCache to cache the responseObject.  Previously the method would be checking for a cached value, but would often not find a previous image because it was not being cached in this method.
Define AF_APP_EXTENSION based on availability with the preprocessor

As seen on Availability.h
Merge pull request #2346 from dtorres/auto-extension-availability

Define AF_APP_EXTENSION based on availability with the preprocessor
[Issue #2346] Refactoring preprocessor directives to remove reference to AF_APP_EXTENSION
[Issue #2346] Adding preprocessor directives around UIApplication network activity indicator
[Issue #2346] Fix preprocessor directive to pass if __has_feature is unavailable
[Issue #2346] Conditionally compile AFNetworkActivityManagerTests
[Issue #2346] Revert commits related to __has_feature(attribute_availability_app_extension)
Merge pull request #2331 from johnbushnell/UIButton-caching-fix

Update UIButton+AFNetworking.m
Declare the init method on subclasses of AFURLResponseSerialization
Update .cocoadocs.yml
Merge pull request #2357 from orta/patch-4

Update .cocoadocs.yml
Merge pull request #1 from AFNetworking/master

update
Document expected session manager usage for background sessions
[Issue #2339] Added lock/unlock to description method
Merge pull request #2371 from getaaron/there-can-be-only-one-background-session

There can be only one background session
Rewriting background session manager warning, duplicating across URL and HTTP session manager
Replace KVO (which was crashing) with swizzling.
Changing validatesDomainName to readonly property

Clarifying documentation and behavior for validatesDomainName
Copy editing header documentation