Simple poc ffmpeg native interface

Mostly infrastructure to support building and loading of the native library.
It seems that the library initialization is not done until use, so this
shouldn't break anything immediately.
Convert Makefile to build.sbt; use tmp library file

Loading a fresh, temporary copy of the library each time seems to fix all the
problems, but it does result in multiple redundant linkages in dev mode.
Cleanup av.c, rename classes, no new development
Extract duration (renamed from length) from uploaded videos

Currently it'll just throw an exception if it can't read the video, or if
something else goes wrong in that whole thing.  No transcoding (yet).