Squashed commit of the following:

commit 6689350d7e4dad6d873c1ed95f0a356e3bfd79d1
Author: Andreas Huber <andih@google.com>
Date:   Tue Jan 19 09:23:02 2010 -0800

    Some tweaks to the SampleIterator.

commit 5638bff6d31442a219806445c3106d47b081fcab
Author: Andreas Huber <andih@google.com>
Date:   Fri Jan 15 14:46:29 2010 -0800

    A much improved implementation of MPEG4 sample table operations through an iterator.