x264: fetch snapshot of HEAD
ffmpeg: fetch snapshot of HEAD
switch to sameersbn/debian:jessie.20140918 baseimage
docker readability updates
use libopus-dev package from the repos
use libvo-aacenc-dev package from the repos
use yasm package from the repos
added install script

the install script installs the required build dependencies and
consequently removes them once the build is complete. As a result the
final docker image is way smaller.