Combine Plugin JS files

I made a small change in the handling of the Plugin JavaScript loading. This way if the DEV_MODE is not on scripts will get combined. This shaves off about 100 requests in my current configuration. And is a considerable speed boost.

One adjustment I can make if there is a need for it is a configuration tag for plugin authors to disable the combining of JS scripts, either for development or perhaps because stuff breaks.
 
Signed-off-by: Chris van Dam <chris@next-heroes.com>