Migrated to AFNetworking 2.2.3.

This pull request replaces: https://github.com/koke/WordPressApi/pull/14

Migrates the project to AFNetworking 2.2.3. The test application was run and no issues where found.

Also:

Increases the deployment target to iOS 6 - AFNetworking 2.2.3 doesn't support anything below that.
Removes the OS X target from the podspec, since it's not building (got errors when running: pod spec lint).
Replaces deprecated methods with their new counterparts (due to the deployment target change).