Add support for watchOS

[daltonclaybrook]

Resolves #388

This PR updates the SDK to include support for watchOS. It also adds a new demo project for Watch that demonstrates the use of the SDK on that platform. The only consequential change to the SDK is the addition of a new type: WatchBackgroundNotifier. The doc comment in that file explains its purpose:

watchOS adds support for background notifications in watchOS 7.0 with WKExtension.applicationDidEnterBackgroundNotification and related notifications. But since this SDK is backward-compatible with watchOS 3.0, these notifications are not available. Instead, the user should implement the appropriate background methods on their Extension Delegate and call the WatchBackgroundNotifier from those methods.

Because this notifier is only used on watchOS, the file structure has been changed slightly to include a Shared folder and a watchOS folder, and the Podspec has been updated to include the appropriate paths for supported platforms. Please let me know if you have another idea about how to solve this problem. Also, feel free to make changes to this branch as you see fit.

[jaeopt]

@daltonclaybrook Thanks for your contribution. We'll take a look at it.
If not done yet, can you sign our CLA?
https://github.com/optimizely/swift-sdk/blob/master/CONTRIBUTING.md

[jaeopt]

@daltonclaybrook The changes and additions for watchOS look great!
One changes I pushed is to change the source structure (bring down "watchOS" codes inside the source module).

[jaeopt]

LGTM