Building tests optional. Default is ON, same behaviour as before.

Hey, I figured I create a pull request here since I liked your fork and the openexr guys seem to be slow.

What do you think of making the tests optional? I've added an option to not build them (default is still ON). I had some problems building the tests on Mac Os 10.9.4 and figured you don't really need the tests if all you care about is the lib files.

cheers