Add support for NSProgress

[abbeycode]

Make URKArchive implement NSProgressReporting on OS X >= 10.11 and iOS >= 9.0, and support implicit progress reporting for earlier OSes.

https://developer.apple.com/videos/play/wwdc2015/232/

Remaining tasks:

• Add NSProgress support to all top-level methods
• For extraction to disk, add passing of the current file info to NSProgress
• Add section to Readme describing sample NSProgress usage and cancellation
• Deprecate the methods that use a progressBlock argument, mentioning migration to NSProgress
• Update documentation