Use googleClosure for js minimization instead of jsMin

Hi Julien,

here is a pull request which fix the problem with js minimization. The pull request has a small change which replace the jsMin minimizer with googleClosure. The later is better and more efficient. 

Apparently the cause of the problem was the following statement from underscore.js:

    var id = '' + ++idCounter;

When minimized with jsMin, it produces the following js:
 
    var id=''+++idCounter;

which is obviously is an invalid statement.