2.1.0 Beta 1

Based on #9, Example here.

• Add new method addBackgroundUploadTask to support background-uploads
• Add new methods flush and reset for an improved resources handling
• Deprecate backgroundDownloadTaskWithURL in favor of addBackgroundDownloadTask
• Move URLSession.Session and URLSession.SessionConfiguration to own namespaces
• Deprecate old single-argument constructor for sessions and session-configurations
• Introduce object-based constructors to incorporate the new namespaces properly