Add support for macOS clients

[leisurehound]

No description provided.

[anantcodes]

The initial support has been added. Please check.

[leisurehound]

I need to back out this change, I failed to realize it was going into the master branch. It should not have been. I will repair the change later today. But macOS impl should be done this coming week.

[anantcodes]

Okay

[leisurehound]

Here is a PR with the macOS support. It passes tests and I'm using it in a client app successfully now. However, my macOS experience is rather limited (i.e. is NSTemporaryDirectory() the best consistent location to cache the local files before upload)? If there are others with more macOS experience who can take a look that would be appreciated.

#3