[Issue #1205] Initial implementation of change to single response serializer
Implementing AFCompoundSerializer
Using compound serializer by default with AFHTTPClientgs
Removing _AFNETWORKING_ALLOW_INVALID_SSL_CERTIFICATES_ backwards compatibility
Executing passed success/failure blocks in content type operations
Executing passed success/failure blocks in content type operations
Make subclasses of AFHTTPSerializer respect HTTPRequestHeaders (fixes #1209)
Merge pull request #1210 from a2/2.0

Make subclasses of AFHTTPSerializer respect HTTPRequestHeaders
Merge branch '2.0-single-response-serializer' into 2.0
Merge branch '2.0' of github.com:AFNetworking/AFNetworking into 2.0
Adding documentation to AFURLSessionManager

Renaming invalidateSessionCancellingTasks -> invalidateSessionCancelingTasks
Minor documentation cleanup
Replacing content-specific response serializers with inherited responseSerializer property
UIImageView responseSerializer -> imageResponseSerializer