Modified OpenCV download script.

* opencv/jamfile:
  * Modified download script because OpenCV source file archive is renamed
    from `opencv-*.tar.gz` to `opencv-*.zip`.
  * Changed the target name that represents OpenCV source trees because
    `README` file has been renamed to `README.md`.
Merge branch 'master' into fix/opencv
Merge pull request #113 from Cryolite/fix/opencv

Modified OpenCV download script.