Make "add licensing" a configurable option

The advagg_js_compress_prep() function adds comment lines at the beginning and end of each compressed Javascript file with links to the original file from which they were derived. This arguable improves GPL compliance of the module. However, the comments also generate some warnings from automated security scans such as Cenzic. To provide flexibility, therefore, the question of whether or not to add the licensing comments ought to be configurable.