Feature/tfdt box

[jensenn]

This PR adds the tfdt box which is necessary for isobmff files

[alfg]

Thank you! Will take a look and test this soon.

[jensenn]

There will likely be another PR for better handling of track fragments. Currently the track functions, sample_offset(), sample_time() and sample_rendering_offset() need some work to better handle track fragments.

I'm also thinking about a good way to handle parsing track fragments if they are from different readers. I am currently using a custom struct that is a cross between io::Cursor and io::Chain as my reader. Perhaps adding a read_fragment(&mut self) function to Mp4Reader.

Again, the end goal is parsing isobmff mp4s that may have the init separate from track fragments.

[alfg]

Sounds good. Thank you!