@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.
There's a lot going on here. Let's split it.

`XAppRequest()` - make sure we have an XAppRequest token first. I think this could potentially be one of the first API calls happening on load. Better to be sure we can do it.
`filterSuccessfulStatusCodes()` - we want to look for HTTP responses with :+1: responses ( 200...299 ) if we don't get one of those, error.
`mapJSON()` - we get a string back from the server, turn it into a JSON dict/array
`subscribeNext()` - this is a subscribe to RAC signals thing, I'll leave that to my buddy @mattt to explain [here](http://nshipster.com/reactivecocoa/)