Optional Security Enhancement

Related to https://github.com/Citytechinc/cq-groovy-console/pull/12  

I added an optional configuration to specify a single group with permission to execute scripts. When the configuration is not specified, no authorization check occurs. This maintains that this change is backwards compatible.

When the group name is specified, the ScriptPostServlet checks to make sure the current user is a member of that group. If the current user is not a member of that group, the servlet returns an error message "Error - you do not have permission to use the groovy console"