fixed opencv source location, 2.4.5 not available on sf.net anymore
also, opencv-2.4.9 is only available as .zip, not as tar.gz
updated opencv source location, 2.3 not available anymore, use 2.3.1 to keep compatibility
Merge pull request #11 from planthaber/opencv

fixed opencv source location, 2.4.5 not available on sf.net anymore