Move convert_to_argb and MJPGToARGB to isolated files.  Move ValidateJPeg to its own file.  Allows jpeg to be not linked in for more applications.
BUG=212
TESTED=manual test by removing mjpeg_decode.cc from gyp file and built/ran unittests
Review URL: https://webrtc-codereview.appspot.com/1310007

git-svn-id: http://libyuv.googlecode.com/svn/trunk@656 16f28f9a-4ce2-e073-06de-1de4eb20be90