Checks to see if os is Windows before launching generated ruby app.

Addresses #35 by checking for Windows and just not launching the app if it is. I used the java.lang.getProperty instead of the rbconfig route, like @enebo advocates in this [SO answer](http://stackoverflow.com/a/13586108/1200100) because to my eye, in this case, the java method actually seems a little cleaner. Since the name it returns for Windows PC's should always contain "win", we can dramatically reduce the number of conditions. If I needed to check for multiple OS's, or if jrubyfx was not already tied to jruby by its very nature, I'd certainly have gone the other way. Certainly willing to do it another way if there are any objections. Also, if anyone has any ideas about fixing the actual cause of the error and not just avoiding it, that would be great to.