Add ability to create OS X application bundle.

Add the ability to create an application bundle under OS X.  Application bundles are the standard method of packaging applications on OS X.  This changeset takes the OpenRocket jar file and wraps it in the standard application bundle format.  The advantage over simply running a jar file in OS X is that the application appears to be a standard OS X application, complete with icon.  In addition, rather than "Startup" showing up as the application menu (that being the name of the main class), the application name OpenRocket shows up as the title of the application menu.  The application bundle is created using jarbundler from http://informagen.com/JarBundler/ (it's licensed under the Apache Software License v2.0).  The mac version of the icn file was created from ./core/resources-src/pix/icon/icon-large-3.xcf.gz because mac icons go up to 512x512 pixels and using the 256 pixel icon showed noticable pixelation.

Some changes that I had to make that surprised me were that the build.properties file in core/resources/ was not properly being referenced in the swing directory.  I moved the property definition of the core directory up to the top and added core.dir as a prefix to the reference of the build.properties file.  If there's something about that I didn't understand then it can be safely ignored as it has nothing to do with application bundles.