Add support for watchOS

[daltonclaybrook]

We need the ability to use the Optimizely SDK from a watchOS app. It appears that this is not currently possible. This should hopefully be a fairly straightforward change considering many of the same Swift/Foundation APIs are available for both iOS and watchOS.

[jaeopt]

@daltonclaybrook We'll take a look. Any size limitation? Our SDK currently about 800KB (400KB compressed).

[daltonclaybrook]

I think that should be fine. I believe the current size limit for watchOS apps is 75 MB.