Fix download link for release 2.4.8

OpenCV version 2.4.8 uses a .zip file release.  This breaks the
download script which expects a .tar.gz.  Modify the script to
support either a tar.gz or .zip file in order to wirk with 2.4.8
Merge pull request #17 from dwalkes/master

Fix download link for release 2.4.8