Added proper checks for observing AFHTTPRequestSerializerObservedKeyPaths

This addresses issues #2284, #2288 and #2289
Merge pull request #2294 from jmkk/dynamic_keypaths_serialization

Added proper checks for observing AFHTTPRequestSerializerObservedKeyPaths
Squashed commit of the following:

commit 78b42ccb1798b9e4c67136f693002cdbb73b7417
Author: TimWatson <tewatson89@gmail.com>
Date:   Mon Sep 15 14:48:52 2014 -0700

    Removed extra parenthesis

commit c9195763295a6939cfc395d1a514500d5ff7d5be
Author: TimWatson <tewatson89@gmail.com>
Date:   Mon Sep 15 12:09:16 2014 -0700

    Switched User-Agent to use the version number instead of the build number

Signed-off-by: Mattt Thompson <m@mattt.me>
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Adding test for HTTPS request
[Podfile] Update to explicitly define sources

Integrate using CocoaPods 0.34
Merge pull request #2328 from kylef/podfile

[Podfile] Update to explicitly define sources
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
fixes KVO crash on ios7