Adding dynamic framework target for Async

Adding a dynamic framework target for better linking into swift projects. 

Now that iOS8 supports dynamic frameworks, this is the ideal way to link in Swift projects. This is not yet supported by Cocoapods, however a [PR](https://github.com/CocoaPods/CocoaPods/pull/2835) for Swift frameworks exists.

The following adds:
- a new xCode project with framework target for iOS
- exposes Async interface to framework
- Adds iOS unit tests to the framework target tests (however these are currently broken both in the framework and the iOS app example)

To dynamic link against the new framework, follow the [instructions here](https://github.com/Alamofire/Alamofire#installation), but reference to Async.framework instead