[WIP] @orta => Basics of transparent self-app-token-fetching API client

There is a call to `.FeaturedWorks` ("/api/v1/sets") in the app delegate as a proof-of-concept. 

The basics of use are, instead of calling `ArtsyProvider.request(.Whatever)` as you normally would, you just call `ArtsyProvider.XAppRequest(.Whatever)`. It'll check if the [XApp token](https://developers.artsy.net/docs/authentication) is present and hasn't expired yet, transparently fetching a new one if required, and signing all non-XApp token requests with the XApp token. 

Work-in-progress because I still want to address: 

- [ ] Needs tests.
- [ ] I don't like the `ReactiveMoyaProvider` extension instance method referencing `ArtsyProvider`, an instance of that class. 

![](http://i.imgur.com/UT69wID.gif)
OR are you saying that if they enter in blank values, they'll still be created with the plugin but they'll be empty? That'd be awesome, but I'll have to modify my code here a bit.