cleanup of include order, std include comments on what function they are for, and guard naming to match folder
BUG=none
TEST=none
Review URL: http://webrtc-codereview.appspot.com/333028

git-svn-id: http://libyuv.googlecode.com/svn/trunk@125 16f28f9a-4ce2-e073-06de-1de4eb20be90