Added support of OpenCV.

* .gitignore: Told to ignore `opencv-*` directory and tarballs.
* bootstrap: Added support of `--enable-opencv`.
* jamroot:
  * Added handling of `--enable-opencv`.
  * Added `<opencv>` and `<opencv-hidden>` features.
* opencv/latest.sh: New. Added script to extract the latest version of
  OpenCV.
* opencv/jamfile: New. Added build script of OpenCV.