Bug 918550 - Update libvpx to 1.3.0

This updates our in-tree copy of libvpx to match the 1.3.0 git
tag (2e88f2f2ec777259bda1714e72f1ecd2519bceb5) libvpx 1.3.0
inclues support for VP9. VP9 support is enabled
but not exposed with this commit.

Our update.sh script is replaced with update.py that can
update the build system to a given git commit.
 - checkout out upstream git
 - create platform dependend config files
 - add/remove changed libvpx files
 - update moz.build
 - warn about new build categories in libvpx