Box Cocoa SDK

- Added a new target to build the SDK into a Cocoa Framework.

- Extracted the UIKit dependent parts of the SDK (the Folder Picker, in particular, and some of the categories), and created a couple classes which are 'Cocoa mirrors' of the iOS specific counterparts. I debated whether to do this with #ifdefs in single files or creating duplicate files: I went with the duplicate file approach for readability, but it means we need to make sure that any changes in, e.g., BoxSDK.m are reflected in BoxCocoaSDK.m.

- Cleaned up the dependencies in the SDK to make sure UIKit wasn't being included where it wasn't needed.

- Added all the BoxSDK tests to the BoxCocoaSDKTests target, and cleaned up the dependencies in the tests to avoid UIKit getting included there as well.

- Fixed a couple of tests with type mismatches that were showing up for OSX, but getting overlooked by iOS.