Implement new class SyncRequest

This class and subclasses will be passed from ObserverService and FileObserver to SynchronizationService
instead of RemoteOperation (UploadFileOperation, RemoveFileOperation, DownloadFileOperation)
The naming is more clear, and ObserverService doesn't have to create the RemoteOperation wich is out of its scope.

The design isn't yet full complete so this Commit might be updated
As it is now, the implementation isn't complete so the build won't succeed.

Changes:
- Add SyncRequest class into models package
- Add UploadRequest as subclass of SyncRequest
- Add DownloadRequest as Subclass of Syncrequest
- Replace ComparableOperation in ObserverService and in SynchronizationService by SyncRequest
- Add a method "CreateRemoteOperation(SyncRequest request" in SynchronizationService