Merge pull request #1 from AFNetworking/master

update
Document expected session manager usage for background sessions
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
Changing validatesDomainName to readonly property

Clarifying documentation and behavior for validatesDomainName
Copy editing header documentation
Merge https://github.com/AFNetworking/AFNetworking