Bug 918550 - Update libvpx to 1.3.0 r=glandium,cpearce

This updates our in-tree copy of libvpx to the
v1.3.0 git tag (2e88f2f2ec777259bda1714e72f1ecd2519bceb5)
libvpx 1.3.0 adds support for VP9. VP9 support is built
but not yet 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