Support Android 6 Permission Model

Currently, the app is built for API level 22 and does not yet support the new permission model of Android 6 (API 23).

This affects at least the permission ACCESS_COARSE_LOCATION. We need to ask for this permission before adding contacts via Bluetooth. For details see #223 (closed).

See https://developer.android.com/training/permissions/requesting.html for more information.