Enabled certificate pinning in demo project
Refactor session task state observing

1) Let's do all the logic in one place, when we setup the delegate. This
   removes having 4 different addObserver's and 4 different removeObserver's.

2) Using selectors for keypaths so we have compile time checking that a `state`
   method exists.
Add support for specifying a response serializer on a per-instance of `AFURLSessionManagerTaskDelegate`
Removed unreachable code
Merge pull request #1843 from philippec/master

Removed unreachable code
Update CONTRIBUTING.md
Update CONTRIBUTING.md
Revived reachable code.
Merge pull request #1845 from jimydotorg/master

Revived reachable code.
[Issue #1843][Issue #1845] Ignoring -Wunreachable-code in AFKeyPathFromOperationState
[Issue #1833] Podspec fix.
[Issue #1772] Updates to 64 bit architecture

Signed-off-by: Mattt Thompson <m@mattt.me>
Merge pull request #1846 from lemonkey/master

[Issue #1833] Podspec fix.
Merge pull request #1784 from jpsim/master

Enabled certificate pinning in demo project
Merge pull request #1723 from blakewatters/feature/allow-per-task-response-serializers

Add support for specifying a response serializer on `AFURLSessionManagerTaskDelegate`
Merge pull request #1805 from kylef/state-observing

Refactor session task state observing
Merge branch 'master' of github.com:AFNetworking/AFNetworking