Added use of AFNetworking to perform network requests.

These changes allow DeploydKit to run over AFNetworking.  The primary reason for this is to enable access to a Deployd server over HTTPS.  The current DeploydKit code uses only synchronous NSURLConnection operations, which as far as I've been able to determine, precludes the use of HTTPS.  Running over AFNetworking should get around this.

I've also created a simple test project to test using these changes at https://github.com/dblapps/DeploydKitTester

To use the changes:

- create an XCode project
- download AFNetworking and DeploydKit.  Put the AFNetworking/AFNetworking directory and the DeploydKit in the same subdirectory within your project directory
- add AFNetworking and DeploydKit to your Xcode project, following the usual instructions for each

The changes in DeploydKit will look for ../AFNetworking/AFNetworking.h.  If that cannot be found, the existing method for network access will be used.