Close the audio codec before closing the format context

Closes #127.
Document the lifetime of FFMS_Frame

There was an offhand mention that the frames are owned by the video
source in the sample code, but not in the obvious place to look for such
things.

Closes #130.
Remove invalid use of assert

The value is checked directly under it, and this is an entirely
valid use-case.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Change C-style includes to proper C++ includes

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Merge pull request #123 from dwbuiten/miscfix

Misc Fixes / Changes