Fixed Negative capturedVideoSeconds Bug

In PBJVision.m, I added an if statement to handle situations where the method capturedVideoSeconds was returning a negative value.
This situation had prevented my multi-view application from recognizing that new video was being captured on a return visit to a view that recorded video.
Merge pull request #122 from eliotw1/fix-video-timestamp-error

Fixed Negative capturedVideoSeconds Bug