Added target for creating osx app bundle

hey ... i've added a target for creating an osx app bundle to "build.xml". i've also added the "appbundler" library to "vendor/", which is required for creating app bundles. the bundle can be created by executing

    ant bundle-phpdbg-ui

the app will be bundled with the JRE found at JAVA_HOME, so that one do not have to install an appropriate JRE on his own. i've currently only tested with osx 10.9 ... and it works like a charm for me (also tested by transferring the created app to a different mac ...). the target takes the ".jar"s from dist/ and the created app will be placed in dist/, too.