Add in support for advagg

During the port to Drupal 7, I modularised a lot of the code, and split out the compilation functions into a separate file so that it can be easily used by other modules.

I have also ensured that the source JS files are kept, with their URLs provided in the optimized JS files, to make sure that all licences are correctly preserved and available.

If you need anything to change in order to make it easier to use, then let me know.