Add new option use_system_js_libraries to control JavaScript minifying and JavaScript libraries used

This branch does not solve issue #199 (closed) but at least it provides a new option use_system_js_libraries allowing the user a better control on which JavaScript libraries are used. When use_system_js_libraries=true, index.php and js-css-minify.sh will use system installed JavaScript libraries; else it will uses the versions provided by MyPhotoShare (By default).