Fixing session cleaup in stopScanning. Now removing inputs, outputs, and moving to a background thread. Previously calling stopScanning would cause a threadlock in -[AVCaputreMetadataOutputInternal dealloc]
Updating session and capture device to be compatible with new discarding logic and hasExistingSession variable.
Checking and setting hasExistingSession in stopScanning
Merge pull request #4 from brandonschlenker/session-cleanup-fixes

stopScanning AVCaptureSession cleanup bug fixes