Download and aggregate JS

Attached patch adds an option to cache locally if the script is external.

The http request is only fired if the file isn't already cached. I stole the function from google analytics which works this way.

I also added a cron hook to ensure that the cache is always primed.

1. This makes sense in general. For some of our use cases in Drupal Gardens it might not actually be an overall performance improvement though? (If all the sites are referencing the same exact file, it allows shared caching of the resource, whereas if they switch to each downloading it locally then they're on their own).
2. Looking at the patch, at first glance I don't see why the cache needs to be primed on hook_cron(), given that javascript_libraries_cache() automatically primes it when the file doesn't exist... Or was the intention here to implement what Peter suggested (automatically force a re-download a couple times a day to check for changes)? In that case, it needs to pass TRUE as the second parameter to javascript_libraries_cache().

3. +function javascript_libraries_cache($location, $sync_cached_file = FALSE) {
   .....
   +    if ($result->code == 200) {
   +      if (file_exists($file_destination)) {
   .....
   +      }
   +      else {
   .....
   +          // Return the local JS file path.
   +          return file_create_url($file_destination);
   +      }
   +    }
   

   Seems like the function should return file_create_url() somewhere in the "if" statement also?