background updates, pull info from config.json

- kill the recursively spawning background thread. (it probably worked to keep the app alive in iOS6, but no longer works in iOS7)
- refactor AppDelegate to record user preferences on app start/background/terminate
- refactor periodically updating probes to pull config info from config.json
- don't include ActivityManagerProbe in ```[OpenSense availableProbes]`` if the phone doesn't support m7

```
notes: 

* I'm starting to shift my efforts to UX design and creating an openPDS connector, so the probe-specific pulls may slow down now. (There are likely to be more integration pull requests)

* I think that the OSActivityManagerProbe is bug-free, but I don't have an iPhone 5S+ phone to fully test it with. Until I get my hands on one, it may remain untested.

* the periodic update probes _could_ inherit from a superclass, but there are only 3 of them... and the additional complexity of managing that just wasn't worth the very minor amount of DRYness that would have resulted

* there's still potential to wake-up the probes on significant location change.  I'm leaving this for another time.


```