Add a debug description to Reachability objects

This prints more info about the object when running `print-object` in the debugger on a Reachability object or when an object is logged to console. I deliberately didn't include every single property about the object; I found these fields to be a good amount of "summary" info at a glance, and one can always query more specific properties about the object if desired.

(Note: there's a new API on `NSObject` for `-debugDescription`, starting with iOS 6.0 and OS X 10.8, but I didn't want to include nine kinds of `ifdef`s for a small "this would be nice" pull request.)