@laurabrown => Get a system time offset from the Artsy servers

For people following along at home, I will inline describe what is going on. 

At an auction we want to ensure that timezones do not become an issue, for example if the artsy servers are on a different time zone to the auction we want to ensure all dates are kept sanely in sync. 

So there's an Artsy time service. I've made a `SystemTime` object that we can use to get the correct date from whenever a date is needed. It knows if it's in sync, if it isn't it returns today's date. This is basically a direct swift port of what we use in Eigen ( the iPhone app ).

@AshFurrow can you give it a one-over too.
@orta We could use an optional here instead of relying on `NSTimeIntervalSince1970` as a sentinel value.