Add experimental OS X support

[chrisvasselli]

• Exclude ALAsset-related code
• Exclude AppToApp authentication
• Add OS X target and scheme, OS X unit test target and scheme
• Updates to handle some differences in keychain behavior on OS X vs. iOS
• Replace UIImage with NSImage on OS X
• Add OS X specific user agent
• Update podspec to support OS X

To do:

• Authentication
• User-friendly names for Mac models to include in user agent

[boxcla]

Verified that @clindsay has signed the CLA. Thanks for the pull request!

[chrisvasselli]

I'm not crazy about all of the #if TARGET_OS_IPHONEs strewn about. I think it might be cleaner to move some things into Mac-specific and iOS-specific files at some point. Or at least factor things in such a way that we don't need to use #if's to block out pieces in the middle of a method. But it does create the simplest diff to understand, so I think it's good to start from this and evolve it afterwards.

[chrisvasselli]

@RicoYao

[chrisvasselli]

@rknottsbox

[rknottsbox]

I agree that the #if TARGET_OS_IPHONE code is a little messy, but it certainly makes it easy to figure out what code is iOS specific.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

:x: Chris Vasselli
:x: rknottsbox

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[stale[bot]]

This issue has been automatically marked as stale because it has not been updated in the last 30 days. It will be closed if no further activity occurs within the next 7 days. Feel free to reach out or mention Box SDK team member for further help and resources if they are needed.

[stale[bot]]

This issue has been automatically closed due to maximum period of being stale. Thank you for your contribution to Box iOS SDK and feel free to open another PR/issue at any time.