Network Reachability

This checks for an available internet connection before attempting an upload. It does not do any sort of quality or speed check.

The Reachability class is taken [directly from Apple](https://developer.apple.com/library/ios/samplecode/Reachability/Introduction/Intro.html#//apple_ref/doc/uid/DTS40007324-Intro-DontLinkElementID_2). Even though that class has more functionality than we really need, I opted to copy/paste it directly, treating it a if it were a blackbox dependency. If anyone has a strong opinion about this, I'm happy to slice and dice it down to just what is required for this piece of functionality. We will likely use more of it later if we want to check WWAN vs wifi for bandwidth limiting purposes.

The unit test only exercises the the logic of calling the `isNetworkConnected` method, not the whole use of the Reachability library. If anyone thinks we should add more, I'm happy to discuss.