Ability to specify custom main class

I've added the ability to specify a custom main class for the MANIFEST.MF, via a new configuration property "grails.plugin.standalone.mainClass". This would be very handy for users that want to perform their own boostrapping of the environment before invoking Tomcat/Jetty. My use case is to read in HTTP/SSL properties from a config.yml, similar to how Dropwizard works, to then pass to Jetty/Tomcat.

I tried to write a unit test for it, but it seems that unit testing gant scripts isn't possible within grails currently. I have manually tested each of the 3 use cases though.

I've also updated the documentation accordingly.