Implementing AggregateFrames Transform

@caotto This is my attempt to implement the transform I described in the [mailing list](https://mail.google.com/mail/u/0/#inbox/14012a31d1cd636f). Does it look correct?

I can't seem to test it, as I can't get the more simple `br -algorithm "Stream([Show(false)+Discard])" -enroll ../data/KTH/vid/boxing` to work for me.

@imaus10 FYI