Various updates

- Add version number
- Clean up typedefs in NetworkManager
- Move HTTP methods out NetworkManager and into category
- Update documentation to describe typedefs
- Remove asserts if op not found (important when we transition to background tasks
- Support background tasks (background property, no max number of operations if background, etc.)
- NB: In background download, didReceiveResponse is not apparently called, so moved initialization of `responseData` to `didReceiveData`.
- Change network queue name to be unique per session manager
- Remove dispatch_sync of various OS completion handlers to main queue
- Add `cancelByProducingResumeData` and `downloadOperationWithResumedData`