Fix of issue #39 for RaspiCam apps

Fixes issues caused when invoking multiple instances of RaspiCam apps.
Prevents the same user from invoking a second instance by checking the lock on a file stored in the user's home directory.
I realise this isn't ideal as it does not prevent multiple users invoking simultaneous instances, or prevent simultaneous access attempts to the camera though MMAL, but I imagine will fix the issue for the majority of users.