Setting default Accept headers for request serializers

Setting default request serializer for AFHTTPClient to JSON
Workaround radar 14783825

Testing for `[[notification object] isKindOfClass:[NSURLSessionTask class]]` should be enough but NSURLSessionTask subclasses lie about their class membership. See http://www.openradar.me/14783825

This commit can be hopefully reverted if rdar://problem/14783825 is fixed before iOS 7 goes public.
Simply test for `respondsToSelector:` instead of `isKindOfClass:` chain
Fixed crash when setting completionQueue and completionGroup.

While copy is best for dispatch_block_t, it is not supported for dispatch_queue_t or dispatch_group_t.
Merge pull request #1244 from tewha/2.0-dispatch-storage

Fixed crash when setting completionQueue and completionGroup.
Fix link AFNetworking logo
Change download link

The download link in Readme should be to the 2.0 branch.
Pass returned disposition instead of forcing NSURLSessionAuthChallengePerformDefaultHandling
Fix broken UIButton KVC code
Merge pull request #1253 from a2/2.0

Fix broken UIButton KVC code
Merge pull request #1252 from getaaron/2.0

Pass disposition instead of forcing NSURLSessionAuthChallengePerformDefaultHandling
Merge pull request #1251 from xHunter/2.0

[2.0] Wrong links in Readme
-Added a user info dictionary to the task did finish notification.
-Added some DRY methods to reduce duplicate logic.
Remove unnecessary #pragma clang diagnostic ignored "-Wassign-enum"
Merge pull request #1266 from 0xced/Wassign-enum-2.0

[2.0] Remove unnecessary #pragma clang diagnostic ignored "-Wassign-enum"
Renaming setResponseSerializers: to setImageResponseSerializer:
Merge pull request #1272 from marcelofabri/2.0

[2.0] Fixing imageResponseSerializer setter on UIImageView+AFNetworking
Refactoring AFURLSessionManager additions
Fixing Clang warning about task being unassigned before being captured in block
Adding completionQueue and completionGroup properties to AFURLSessionManager