Fixes bug introduced in #400, adds button to notification.

PR #400 introduced a sneaky race-condition bug. It would try to destroy scanner service BEFORE all the resources from the "downstream" scanners could be turned off by the Looper. Memory leaks and fatal errors ensure. It would only trigger on a low battery intent when there was no client bound...

* new button to turn off scanning with main activity not-running
* fix to the aforementioned bug

One broadcast receiver can handle both intents.
Merge pull request #467 from tamcap/bugfix+notification_button

Fix scanner service race condition; Add stop scanning button to notification (@tamcap)