Update COPYING notice to further clarify our conventions regarding copyright...

Update COPYING notice to further clarify our conventions regarding copyright and multiple authors; update Apache headers to point to COPYING notice.

git-svn-id: https://svn.code.sf.net/p/kaldi/code/trunk@2988 5e6a8d80-dfce-4ca6-a32a-6e07a63d50c8