add opencv3 as devel build

Adding opencv3 as the devel build for OpenCV.
The devel block is not supported in the install method AFAIK. If that argument conflicts with the existing gstreamer arg you might want to guard with 
```
if build.devel?
  args << "-DWITH_GSTREAMER_0_10=" + ((build.with? "gstreamer") ? "ON" : "OFF")
else
  args << "-DWITH_GSTREAMER=" + ((build.with? "gstreamer") ? "ON" : "OFF")
end