ARGBSepia planarfunction for converting a region of ARGB image to Sepia tone.  ARGBGray optimized weaving of alpha value.  551 ms from 568 ms.
BUG=none
TEST=libyuv_unittest --gtest_filter=*ARGBSepia*
Review URL: https://webrtc-codereview.appspot.com/573008

git-svn-id: http://libyuv.googlecode.com/svn/trunk@270 16f28f9a-4ce2-e073-06de-1de4eb20be90