Added support to sideload the javaagent

With Premain-Class defined I could start the JVM up with -javaagent:quasar.jar, but some times its nicer to pull this in after the JVM is already started (running main in IDE without needing to configure javaagent).  This patch adds Agent-Class to the manifest and makes agentmain delegate to premain.

Tested with the following code in my main method:

    ...
    public static void main(String[] args) {
        loadAgent();
    ...
    public static void loadAgent() {
        System.out.println("dynamically loading javaagent");
        String nameOfRunningVM = ManagementFactory.getRuntimeMXBean().getName();
        int p = nameOfRunningVM.indexOf('@');
        String pid = nameOfRunningVM.substring(0, p);

        try {
            VirtualMachine vm = VirtualMachine.attach(pid);
            final File agentJar = new File("../quasar/quasar-core/build/libs/quasar-core-0.6.0-SNAPSHOT.jar");
            if(!agentJar.exists()) error("Unable to locate agent jar!");
            vm.loadAgent(agentJar.getAbsolutePath(), "");
            vm.detach();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }