Minimal suport to enable forked mode debugging in GGTS.

This is just to give an idea of what's needed. I didn't quite get things to work properly in terms of keeping any memory related settings that may be provided in the user's BuildConfig.groovy.

I.e. we would like to override the 'debug' and 'debugArgs' via system properties but keep the settings otherwise provided by the user.

I imagine that what's coded up here will revert memory settings to the Grails defaults and forget about any potential settings in BuildConfig.

Related issue in Jira:
http://jira.grails.org/browse/GRAILS-9836