Core Location support

:warning: **Not Ready to Merge** :warning: 

First thing first, this could use a lot more tests. Testing this proves to be more effort than it might be worth, SURPRISE. First, apps need to be authorized to access the location, which means any tests won't run out of the box, the user needs to provide authorization. This is doable on OS X, but on iOS this is actually impossible with the "logic" test target. Might be possible with the "application" test target, but I haven't tried. Right now, the basic test I wrote is conditional disabled, it needs a `RAC_ENABLE_CORE_LOCATION_TESTS` before it will run.

This has support for the full location update stream, `-rac_activeLocationUpdatesSignal`, but I'm thinking a signal for just significant location updates might be useful too.