XCode changes

There were some issues with merging because of line endings so the different commits had to be compressed into one to make things sane. These changes were to get pubnub to compile as a static library in xcode and might not be relevant enough to have in the master branch, but take them as you will.

The individual commit messages were as follows:
Add missing includes and class forward declarations
Add Xcode project for compiling PubNub as iOS static library
Add include for <Availability.h>
    - This is needed in order for __IPHONE_OS_VERSION_MIN_REQUIRED to be defined