Native package : adds the ability to easily create native packages and installers using JavaFX packaging toolset

So, with this you'll be able to use JavaFX's Ant Tasks to package native installers and apps. Couple of points:

- As of now, this only works with JDK8. The packager with JDK7 isn't as flexible, and demands that it creates the jar itself, and a bunch of other stuff. They were going to fix this in update 10 but it didn't make it in, so right now, JDK8 is the only place they are available. I've been in touch with some people working with Oracle to put these new Ant Tasks into the Maven Central repo. Specifically what we need is ant-javafx.jar. This is supposed to happen in the next few weeks. When it does, I'll update it so that we just grab the tasks from there and use those, and then it won't matter what JDK you are using.

- I wasn't sure about either adding this on to the jrubyfx-jarify tool or making it into its own executable. In the end I went with adding on to the jrubyfx-jarify tool. If this is undesired I will be happy to change it.

- The JavaFX packaging tasks have literally hundreds, maybe thousands of options. I went with an extremely limited set of these, and in places made choices for the user, for the sake of simplicity. We can fill these out over time if desired. I also mentioned in the README the option of making your own rake tasks if you need more flexibility. The wiki page it points to is currently a stub. I'll start filling that page in tonight, but wanted to have the link in place when I pushed.

- I've tested this on Ubuntu and Windows 7. I don't have access to OSX, so couldn't test it there. 

This is my first pull request on a ruby project, so I sincerely welcome your feedback and will be happy to make any requested changes. Thanks.