Optionally dropping frames

My application needed to create a "slideshow" of three frames to be shown with one second each and those frames should be captured one second after the previous. I "achieved" it with the following: 
```
recordSession.suggestedMaxRecordDuration = CMTimeMake(299, 100)
recordSession.videoMaxFrameRate = 1
recordSession.videoTimeScale = 3
```
However, it captured 3 frames, extended their output length to 1 second, and thus, created a 3 second video with three frames of one second each, however, the recording took a fraction of a second. The frames weren't being captured with one second distance of each other. 

My solution adds the boolean property `shouldDropFrames` to `SCRecordSession`. It's default value is `YES`, so nothing needs to be changed for the default behavior to continue, however, if this property is set to `NO`, the session will drop the frames until the full length of the recording is over.

I had to customize the framework in order to achieve what I needed. I do realise this behavior may not be intended, but, nevertheless, I thought it would be interesting to submit a pull-request for your appreciation.