Trigger flash with physical volume button

Currently the observer on
AVSystemController_SystemVolumeDidChangeNotification is firing much too
often; possibly each time the audio session is started. Need to find a
workaround.  Possibly http://stackoverflow.com/a/17569601/72
Change constant variable name
Add minimum time threshhold after which volume button is active

Works around issue where subsequent audio sessions would trigger volume
notification immediately.
Merge branch 'feature/volume-up-triggers-capture-14' into develop

Fixes #14